Set TextEdit to open a specific file by default upon launch

Save the file, and then simply leave it open - don't close that file window - when you quit the TextEdit app.

The next time you launch TextEdit, it will startup and reopen that document.

Alternatively, you can simply double-click the file, wherever you have save it, to open it first thing when TextEdit launches. Maybe that file is one that you might add to your Dock?

Use the Dock on Mac - Apple Support

You're welcome.