Payload supports TypeScript natively, and not only that, the entirety of the CMS is built with TypeScript. To get started developing with Payload and TypeScript, you can use one of Payload's built-in boilerplates in one line via create-payload-app:
Pick a TypeScript project type to get started easily.
It's also possible to set up a TypeScript project from scratch. We plan to write up a guide for exactly how—so keep an eye out for that, too.
Payload exports a number of types that you may find useful while writing your own plugins, hooks, access control functions, custom views, GraphQL queries / mutations, or anything else.