This research study is enrolling participants that have been diagnosed with advanced gastric cancer (a disease in which cancer cells form in the lining of the stomach) or gastroesophageal junction cancer (a type of cancer forming in the area where the tube that connects the mouth and the stomach join together) and have not received prior treatment for advanced cancer and the tumor has already tested positive for fibroblast growth factor receptor 2b (FGFR2b) expression. FGFR2b is a protein that possibly causes cells to grow more quickly and multiply abnormally. The purpose of this study is to see if bemarituzumab is safe, tolerable, and effective when administered along with mFOLFOX6 (a combination of 5 fluorouracil [5 FU], oxaliplatin and leucovorin) and nivolumab and whether it causes any side effects. Bemarituzumab is not approved by the Food and Drug Administration (FDA). Participants will be pre-screening to see if they qualify to participate in Part 2. During the pre-screening portion, participants will provide either archived tissue or a fresh tumor sample in order to test for FGFR2b proteins. If the test results show an excessive amount of FGFR2b proteins in the tumor, participants may be asked to consider participating in the Part 2 study involving bemarituzumab, an investigational drug for people with advanced gastric or gastroesophageal junction cancer. Participants in this study will receive either bemarituzumab with mFOLFOX6 and nivolumab or placebo with mFOLFOX6 and nivolumab. The placebo will look like bemarituzumab but it will not contain active ingredients and has no effect on participants' condition. This study involves pharmacokinetic testing which is a blood test to determine how much study drug is in the blood. Bemarituzumab is not approved by the Food and Drug Administration (FDA). Participants could be in this study for up to 40 months and up to 57 visits.

The purpose of this study is to evaluate investigational treatments (study drug) for unresectable metastatic colorectal cancer to determine if any of these study treatments improve overall survival as compared to standard treatments. The goal is to determine the optimal dose level, safety, and tolerability for the study drug ABBV-400 in combination with fluorouracil, folinic acid, and bevacizumab. The study consists of two stages, Stage 1 - Safety Lead-In Dose Escalation and Stage 2- Dose Optimization. ABBV-400 is not FDA approved. Bevacizumab is FDA approved by the U.S. Food and Drug Administration (FDA) to treat various cancers but may not be approved for your type of cancer. Treatment for this study may be up to 3 years. The procedures include blood and urine samples, questionnaires, infusions, and CT scans. Risks include diarrhea, nausea, vomiting, fatigue, and numbness, tingling, or weakness in arms and legs. You may or may not receive a direct benefit from participating in this trial, however, information learned from the trial may help other people in the future.