It depends on the kind of company, and the context. An ethicist at a company doing drug/medical research probably mostly oversees that the company's research practices are legal and safe (and that participants give informed consent).

At typical corporation, something like a Chief Ethics and Compliance Officer (CECO) would be largely focused on legal compliance - making sure company personnel aren't engaged in bribery, fraud, insider trading etc. The value of this role (and the medical example) is to prevent lawsuits and/or prosecution.

For a company like Meta, an ethicist is likely concerned with the inherent ethical concerns of running a social media company, which impacts the mental health of its users, many of whom are minors. For AI companies, the ethics of releasing models that are capable of doing dangerous things. In name, the job probably involves giving recommendations about how to ethically implement features in the platform to minimize harm. In practice, the job probably accomplishes very little.