A Clinical Study to Test if an Investigational Treatment Called BNT326 is Safe and Potentially Beneficial When Used Alone or in Combination With Other Investigational Treatments Such as BNT327, for People With Advanced Malignant Tumors

Both parts (Part 1 and Part 2) will start enrolling study participants independent of each other. In Part 1, participants with histologically or cytologically confirmed advanced solid tumors will receive BNT326 monotherapy in the following cohorts: * Cohort 1A: Cutaneous melanoma (second-line or higher treatment \[2L+\]). * Cohort 1B: Actionable oncogenic alterations (AGA)-negative non-small cell lung cancer (NSCLC) 2L+. * Cohort 1C: Epidermal growth factor receptor mutated (EGFRm) NSCLC 2L+. * Cohort 1D: Rare melanoma (acral/uveal/mucosal melanoma) 2L+. * Cohort 1E: Other advanced solid tumors 2L+. * Cohort 1F (drug-drug interaction \[DDI\] Cohort): Advanced solid tumors. * Cohort 1G: Cervical cancer 2L+. In Part 2, BNT326 will be studied as monotherapy or in combination with other immunotherapeutic agents. The first combination treatment will be BNT326 with BNT327 (also known as pumitamig or PM8002). The following cohorts are planned: * Cohort 2A: BNT326 + pumitamig for cutaneous melanoma 2L+. * Cohort 2B: BNT326 + pumitamig for human epidermal growth factor receptor 2 (HER2)-negative breast cancer 2L+/first line treatment (1L). * Cohort 2C (Optional): BNT326 + pumitamig for cutaneous melanoma first-line or higher treatment (1L+). This cohort may be added if BNT326 + pumitamig for cutaneous melanoma 2L+ is tolerated, and shows signs of efficacy. * Cohort 2D: BNT326 + pumitamig for gastric cancer (GC)/gastroesophageal junction cancer (GEJC) 2L+. * Cohort 2E: BNT326 + pumitamig for colorectal cancer 2L+. * Cohort 2F: BNT326 + pumitamig for cervical cancer 2L+. Participants in Cohorts 1A, 1B, and 1C (dose optimization cohorts) will be randomized to one of two dose levels (DLs) of BNT326 in a 1:1 ratio. An additional, non-randomized dose level arm (DL3) is included in Cohort 1A. During the dose randomization phase of Cohorts 2A and 2B, participants will be randomized 1:1 to one of two combination DLs. During the randomized dose optimization and contribution of components of Cohorts 2D and 2E, participants will be randomized in a 1:1:1 ratio to one of three treatment arms (BNT326 monotherapy at DL2 or BNT326 DL2 with pumitamig \[DL1 or DL2\] combination treatment). An additional, non-randomized dose level arm is included in Cohorts 2D and 2E (BNT326 DL3 in combination with pumitamig DL1 or DL2). No randomization is planned for Cohorts 1D, 1E, 1F, 1G, 2C and 2F. The study will consist of a screening period, a treatment period, a safety follow-up period, an efficacy follow-up period, and a long-term survival follow-up period. Study treatment will be continued for up to 24 months or until disease progression, withdrawal of consent, termination of the study by the sponsor, or unacceptable toxicity. If a participant continues to derive clinical benefit at the end of study, study treatment continuation can be considered case-by-case after discussion with the sponsor's medical monitor. For each participant, the treatment and follow-up periods are projected to be completed within \~38 months (Part 1) and \~48 months (Part 2), unless participants are continuing to benefit from treatment per investigator's recommendation and upon sponsor approval.