This phase II/III trial compares the effect of immunotherapy with atezolizumab in combination with standard chemotherapy with a platinum drug (cisplatin or carboplatin) and etoposide versus standard therapy alone for the treatment of poorly differentiated extrapulmonary (originated outside the lung) neuroendocrine cancer that may have spread from where it first started to nearby tissue, lymph nodes, or distant parts of the body (advanced) or that has spread from where it first started (primary site) to other places in the body (metastatic). The other aim of this trial is to compare using atezolizumab just at the beginning of treatment versus continuing it beyond the initial treatment. Immunotherapy with monoclonal antibodies, such as atezolizumab, may help the body's immune system attack the cancer, and may interfere with the ability of tumor cells to grow and spread. Cisplatin and carboplatin are in a class of medications known as platinum-containing compounds that work by killing, stopping or slowing the growth of cancer cells. Etoposide is in a class of medications known as podophyllotoxin derivatives. It blocks a certain enzyme needed for cell division and DNA repair, and it may kill cancer cells. Giving atezolizumab in combination with a platinum drug (cisplatin or carboplatin) and etoposide may work better in treating patients with poorly differentiated extrapulmonary neuroendocrine cancer compared to standard therapy with a platinum drug (cisplatin or carboplatin) and etoposide alone.

Researchers are looking for new ways to treat types of breast cancer that are both:

* High-risk, which means the cancer may have a higher chance of getting worse or coming back after treatment
* Early-stage, which means the cancer is in the breast or the lymph nodes around the breast The 2 types of breast cancer in this study are triple-negative breast cancer (TNBC) and hormone receptor (HR)-low positive/human epidermal growth factor receptor-2 (HER2) negative breast cancer. These cancers have zero or a low amount of a protein called HER2 and other proteins that attach to the hormones estrogen or progesterone.

Sacituzumab tirumotecan (also known as sac-TMT or MK-2870), the study medicine, is a type of targeted therapy. A targeted therapy is a treatment that works to control how specific types of cancer cells grow and spread.

The main goals of this study are to learn if people who receive sac-TMT, pembrolizumab, and chemotherapy:

* Have fewer cancer cells found in the tumors and lymph nodes removed during surgery compared to those who receive only pembrolizumab and chemotherapy
* Live longer without the cancer growing, spreading, or coming back compared to people who receive only pembrolizumab with chemotherapy

The main purpose of this study is to find out whether the study drug, LY4050784, is safe, tolerable and effective in participants alone or in combination with other anticancer agents. In addition, with locally advanced or metastatic solid tumors with a BRG1 (Brahma-related gene 1, also known as SMARCA4) alteration who have previously received, do not qualify for, or are refusing standard of care treatments, or there is no standard therapy available for the disease. The study is conducted in two parts - phase Ia (dose-escalation) and phase Ib (dose-optimization, dose-expansion). The study will last up to approximately 4 years.