* INCLUSION CRITERIA:
* Relapsed or refractory solid tumor malignancies for whom standard measures do not exist or are no longer effective. Must have histologic confirmation of original diagnosis or relapse.
* Participants must have evaluable (measurable or not measurable) disease.
* Part B only: Participants must:
* be willing to undergo mandatory pre- and post-treatment tumor biopsies. Tumor tissue should either be taken from non-target lesions or from target lesions where sampling can be done without impacting lesion measurement
and
--have disease amenable to biopsy to allow to perform pre- and post-tumor biopsies.
* Participants with treated brain metastases are eligible if follow-up brain imaging after central nervous system (CNS)-directed therapy and close monitoring for over 3 months shows no active progression.
* At least one prior cancer treatment when upfront standard therapy exists. Note: There is no limit to the number or type of prior treatment regimens.
* The indicated time must have elapsed since any systemic anti-cancer therapy prior to leukapheresis.
* Age \>= 18 years.
* Eastern Cooperative Oncology Group (ECOG) performance status \<= 2.
Note: Participants who are unable to walk because of paralysis, but who are able to maintain supine position independently in a wheelchair, will be considered ambulatory for the purpose of performance status.
* Participants must have adequate organ and marrow function as defined below:
* Peripheral absolute neutrophil count (ANC) \>= 1000/mm\^3
* Platelet count \>= 100,000/mm\^3
* Hemoglobin (Hgb) \>= 8 g/dL (transfusion independent)
* Prothrombin time (PT) \<= 1.5 X institutional upper limit of normal (ULN) (Part B only, participants undergoing biopsy)
* Creatinine clearance \>= 60 mL/minute/1.73m\^2 (calculated by the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) formula)
* Aspartate Aminotransferase (AST) \<= 3 X institutional ULN
* Alanine Aminotransferase (ALT) \<= 3 X institutional ULN
* Total bilirubin \<= 1.5 institutional ULN. Note: In the case of Gilbert's syndrome total bilirubin \<= 3 X ULN
* Serum albumin \>= 2.7 g/dL
* Ejection fraction of \>= 45% and no evidence of hemodynamically significant pericardial effusion as determined by an echocardiogram (ECHO).
* QTc interval \< 480 msec
* Oxygen saturation \>92% on room air at rest
* Participants with a clinical history of prolonged smoking (\>= 10 pack-years), lung disease, or current or recent history of respiratory symptoms must have a forced expiratory volume in the first second (FEV1) \> 50%.
* Participants seropositive for human immunodeficiency virus (HIV) must have an undetectable HIV viral load.
* Participants seropositive for Hepatitis C virus (HCV) must have an undetectable HCV viral load
* Participants positive for Hepatitis B surface antigen (HbsAg) must have an undetectable Hepatitis B virus (HBV) viral load.
* Women of childbearing potential (WOCBP) must agree to use highly effective contraception (hormonal, intrauterine device \[IUD\], abstinence, surgical sterilization) at the study entry and up to 12 months after the last dose of combined chemotherapy.
Note: WOCBP is defined as any woman who has experienced menarche and who has not undergone successful surgical sterilization or who is not postmenopausal.
Men able to father a child must agree to use a highly effective method of contraception (surgical sterilization, abstinence or man may request that partner uses the highly effective form of contraception to fulfill this requirement) at the study entry and up to 7 months after the last dose of study drugs. Men able to father a child must not freeze or donate sperm within the same period.
* Nursing participants must be willing to discontinue nursing from study treatment initiation through 12 months after the last dose of the study drug(s).
* Ability and willingness of participant to enroll on protocol 15-C-0028, Follow-Up Evaluation for Gene-Therapy Related Delayed Adverse Events after Participation in Pediatric Oncology Branch Clinical Trials after 5 years.
* Ability of the participant to understand and the willingness to sign a written informed consent document.
EXCLUSION CRITERIA:
* Participants with history of primary CNS tumors or leptomeningeal disease.
* History of allergic reactions attributed to compounds of similar chemical or biologic composition to cyclophosphamide, fludarabine, IL-12, or other agents used in the study.
* Concurrent untreated opportunistic infections as evidenced by history, blood test or imaging at screening.
* Active systemic infections requiring anti-infective treatment.
* Any form of primary immunodeficiency (such as Severe Combined Immunodeficiency Disease) or secondary/acquired immunodeficiency requiring steroids or other non-steroid immunosuppressive agents.
* History of clonal hematopoiesis of indeterminate potential (CHIP) or myelodysplasia/myelodysplastic syndrome (MDS) or monoclonal gammopathy of undetermined significance (MGUS).
* Participants with symptomatic pleural effusions requiring intervention or with recent history (within 3 months) of pleural effusions that required intervention.
* Participants with ischemic symptoms (may include chest pain/pressure, shortness of breath, nausea, vomiting, sweating, and/or pain in the neck, shoulder, jaw or arm) confirmed by stress test OR a history of coronary revascularization (unless the participant has a normal cardiac stress test after revascularization and within 12 months prior to leukapheresis).
* Any form of diagnosed autoimmune disease requiring immune suppression as well as participants with active autoimmune skin diseases such as psoriasis or any history of active systemic autoimmune disease (e.g., Crohn's, rheumatoid arthritis, systemic lupus) requiring systemic immunosuppression/systemic disease-modifying agents within the last 2 years. Exceptions will be allowed for vitiligo and hypothyroidism that has been stable on thyroid replacement medications for \> 6 weeks prior to leukapheresis.
* Any participant who developed autoimmunity (\>= grade 3 per CTCAE v. 6.0) with checkpoint inhibitor use. Note: Exceptions will be allowed for vitiligo and hypothyroidism that has been stable on thyroid replacement medications for \> 6 weeks prior to leukapheresis.
* Participants who have a major surgical procedure, other than for diagnosis, within 4 weeks prior to leukapheresis or anticipated to need a major surgical procedure during the study.
* History of prior solid organ transplantation.
* Participants that require urgent therapy due to tumor mass effects or spinal cord compression within 2 weeks prior to leukapheresis.
* Any investigational therapy within 2 weeks prior to leukapheresis.
* Pregnancy confirmed with Beta-human chorionic gonadotropin (Beta-HCG) serum or urine pregnancy test in WOCBP at screening. Note: In case of a suspected false-positive serum or urine test result, additional evaluation must be done to rule out pregnancy.
* Uncontrolled intercurrent illness or medical condition(s) evaluated by medical history, physical exam, or situations that would unacceptably increase risk for the participant or impair the ability to evaluate the endpoints of the study.
