A Study of BMS-986504 With Standard-of-Care Therapy for People With Solid Tumor Cancer

Inclusion Criteria: * Documentation of Disease: * Patients must have pathologic confirmation of one of three diseases: * Diffuse pleural mesothelioma (DPM) * Gastroesophageal carcinoma (GEC) including adenocarcinoma or squamous cell carcinoma of the esophagus, gastroesophageal junction, or stomach. * PD-L1 CPS ≥1 (using clone 73-10, DAKO) * HER2 overexpression negative (using clone 4B5, Ventana): HER2 IHC 0-1+, or HER2 2+ with ISH showing HER2:CEP17 ratio \<2 and average HER2 copy number \<6.0 signals/cell * Urothelial carcinoma (UC) * Archival tissue is acceptable; diagnosis may be confirmed at any time before or within 28 days of treatment initiation * Metastatic or advanced/unresectable disease: * For Diffuse Pleural Mesothelioma (DPM) and Gastroesophageal Carcinoma (GEC )cohorts: no prior systemic treatment for metastatic disease * Patients with metastatic disease after treatment for localized GEC may have received prior systemic therapy (chemotherapy and/or chemoradiation) if \>6 months have elapsed between the end of therapy and registration. * One prior cycle of standard-of-care therapy alone without BMS-986504 or other MTAP inhibitors (ipi/nivo for DPM, FOLFOX + nivo for GEC) is acceptable with PI approval. * For UC cohort: must have received at least 1 prior line of treatment without prior gemcitabine (prior tx with Gem+Platinum in the perioperative setting is permitted if at least 12 months have elapsed from trial enrollment) * Patients with recurrent disease within 1 year of completion of prior perioperative systemic therapy are eligible with PI approval. * One prior cycle of standard-of-care therapy alone with gemcitabine + platinum, without BMS-986504 or other MTAP inhibitors, is acceptable with PI approval. * Confirmation of MTAP deletion by either IHC or NGS: * MTAP deletion must be detected by either IHC and/or NGS (including FACETS), done on tumor tissue (not blood): * IHC (using antibody 1813, NBP2-75730, Novus Biologicals)30 * IHC staining showing loss of MTAP expression * Tissue-based NGS options * MSK-IMPACT version 7 or beyond showing homozygous MTAP copy number loss * FACETS showing homozygous deletion * Other CLIA-approved commercial Template Version: 1-21-25 * Full report must be available for review and confirmation * Cases with discordant results between NGS and IHC, in which one test shows MTAP loss/MTAP del and the other shows MTAP intact, are acceptable with PI approval * Measurable disease per RECIST v 1.1 (or, for DPM cohort, by either RECIST v 1.1 or modified RECIST \[mRECIST\] for mesothelioma31) * No contraindications to receiving other standard-of-care agents per package inserts (and see Appendix IV), and per the discretion of the PI: * DPM: Ipilimumab + nivolumab * GEC: FOLFOX (5-FU, leucovorin, and oxaliplatin) + nivolumab * UC: Gemcitabine + platinum (carboplatin or cisplatin) * Age ≥ 18 * KPS ≥ 70/ECOG \<1 * Reproductive Status: * Female participants of child-bearing potential (as assigned at birth) must have a negative highly sensitive urine or serum (as required by local regulations) pregnancy test (minimum sensitivity 25 IU/L or equivalent units of human chorionic gonadotropin) within 24 hours prior to the start of study intervention. o If a urine test cannot be confirmed as negative (eg, an ambiguous result), a serum pregnancy test is required. In such cases, the participant must be excluded from participation if the serum pregnancy result is positive. * If a urine test cannot be confirmed as negative (eg, an ambiguous result), a serum pregnancy test is required. In such cases, the participant must be excluded from participation if the serum pregnancy result is positive. * Female participants of child-bearing potential (as assigned at birth) are not permitted to use hormonal contraceptive methods alone as a highly effective method of contraception and must use an additional non-hormonal highly effective method of contraception (with a failure rate of \< 1% per year)during the intervention period and for 14 months (for females) after the last dose of study intervention (or longer if required by institutional guidelines) * Female participants of child-bearing potential (as assigned at birth) must also agree not to donate eggs (ova, oocytes) for the purpose of reproduction for the same period. * If needed, these participants should be advised to seek advice about egg donation and cryopreservation of germ cells before treatment. * Female participants (as assigned at birth) are deemed to be without child-bearing potential if they meet one of the following criteria: * Postmenopausal for at least 1 year before the screening visit * Permanently sterile (undergone a hysterectomy, bilateral salpingectomy or bilateral oophorectomy) with surgery at least 1 month before the first dose of study drug or confirmed by follicle stimulating hormone (FSH) test \> 40 mIU/mL and estradiol \< 40 pg/mL (\<140 pmol/L) * Male participants (as assigned at birth) will be required to always use a latex or other synthetic condom during any sexual activity (eg, vaginal, anal, oral) with a female of childbearing potential, even if the participant has undergone a successful vasectomy or if the partner is pregnant or breastfeeding. Male participants (as assigned at birth) should continue to use a condom during the intervention period and for at least 11 months after the last dose of study intervention (or longer if required by institutional guidelines). * Male participants must refrain from donating sperm during the intervention period and for at least 11 months after the last dose of study intervention (or longer if required by institutional guidelines). * If needed, male participants should be advised to seek advice about sperm donation and cryopreservation of germ cells before treatment. * Individuals of child-bearing potential who are partners of male participants should be advised to use a highly effective method of contraception during the intervention period and for