Primary: -Safety run-in part To confirm the recommended phase 2 dose (RP2D) of each investigational arm (NIS793 or NIS793+tislelizumab or any investigational drug(s) in combination with SOC) (SOC: either modified FOLFOX6 + bevacizumab or FOLFIRI+ bevacizumab chemotherapy regimen) -Expansion part: To evaluate preliminary efficacy of each investigational arm (NIS793 or NIS793+tislelizumab or any investigational drug(s) in combination with SOC anti-cancer therapy) versus control arm (SOC anti-cancer therapy) in terms of progression-free survival (PFS) by Investigator's assessment per RECIST 1.1 Secondary: -Safety run-in part: To characterize the safety and tolerability of each investigational arm (NIS793 or NIS793+tislelizumab or any investigational drug(s) in combination with SOC) -Preliminary anti-tumor activity of each investigational arm (NIS793 or NIS793+tislelizumab or any investigational drug(s) in combination with SOC) -To characterize the pharmacokinetics (PK) of each investigational arm (NIS793 or NIS793+tislelizumab or any investigational drug(s) in combination with SOC) -To characterize the immunogenicity of components of each investigational arm (NIS793 or NIS793+tislelizumab or any investigational drug(s) in combination with SOC) -Expansion part: To evaluate and compare efficacy of each investigational arm (NIS793 or NIS793+tislelizumab or any investigational drug(s) in combination with SOC) and control arm in terms of ORR, DCR, DOR, TTR by Investigator's assessment per RECIST 1.1 -To evaluate and compare the overall survival (OS) of each investigational arm and control arm (NIS793 or NIS793+tislelizumab or any investigational drug(s) in combination with SOC) -To compare efficacy: PFS, ORR, DCR, DOR, TTR by Investigator assessment per RECIST 1.1 and OS between investigational arm #1 (NIS793 in combination with SOC) and investigational arm #2 (NIS793+tislelizumab in combination with SOC) -Safety and tolerability -To characterize the PK of each investigational arm (NIS793 or NIS793+tislelizumab or any investigational drug(s) in combination with SOC)and control arm -To characterize the immunogenicity of components of each investigational arm (NIS793 or NIS793+tislelizumab or any investigational drug(s) in combination with SOC) and control arm Exploratory: -To explore health-related quality of life and other patient-reported outcomes in each investigational arm (NIS793 or NIS793+tislelizumab or any investigational drug(s) in combination with SOC) and control arm (Expansion part only) -To explore the association of molecular alterations in tumors, tumor subtypes and/or consensus molecular subtypes (CMS), and soluble markers (e.g., ctDNA, cytokines) with clinical efficacy -To explore changes in biomarkers of the immune microenvironment as well as blood derived biomarkers upon treatment and investigate the relationship with clinical response and resistance to each combination arm with SOC anti-cancer therapy -Relationship between the PK of components of each investigational arm and control arm, and the pharmacodynamics (PD) biomarkers, safety and efficacy parameters -To explore impact of immunogenicity on PK, safety and efficacy of each investigational arm (NIS793 or NIS793+tislelizumab or any investigational drug(s) in combination with SOC) and control arm components -To assess the preliminary anti-tumor activity of investigational drug(s) in combination with SOC using Immune Response Evaluation Criteria in Solid Tumors (iRECIST) (only for NIS793+tislelizumab in combination with SOC)