This study is the first time the drug BNT3214 (also referred to as PM8102) will be tested in people. It is designed to find out if the drug is safe and how well it works for adults with advanced solid tumors. The study will have three parts. The first two parts (Parts A and B) will focus on testing different amounts of BNT3214 to figure out the best and safest dose. The third part (Part C) will test selected doses of BNT3214 in multiple types of cancer.