Payload abstracts a lot so that if all you know is JS you can build cool things. I'll comment on each of the things you listed below.
node - not much to learn here really
Express - Pretty simple, just knowing how it works could be helpful for troubleshooting issues in a project or extending your app outside of Payload
Typescript - This is what you should focus on if you have only done JS, TS is a big improvement and will serve you well in your dev career.
React - Knowing React will be helpful if you want to customize the admin UI or build frontend websites using Next.js or plain React.
Mongodb - There isn't much to know here, Payload abstracts the DB so much that you can get by with very little knowledge here to be honest. Also other database support is coming very soon if you're wanting to use SQL.