What should I do if the company writes someone else's name when applying for a patent after the inventor leaves the company? How can I change it?

The patentee is the applicant, that is, your company, and has nothing to do with the inventor. The inventor has no right to this. As for the distribution of benefits, the inventor and the applicant shall negotiate to solve it. Generally speaking, the inventor will get a part of the reward from the invention.

3. It is best to sign a certain agreement, compensation agreement and so on with the company when the design is completed.

There is no point in inventing change without changing it. Unless it's the applicant.