https://strapi.io/documentation/developer-docs/latest/concepts/customization.html#plugin-extensions
Any file that you create in ./extensions/{plugin-name}/ will overwrite the original file from ./node_modules/strapi-plugin-{plugin-name}/. So if you want to modify something in a plugin you just copy the original file into extensions folder and modify it. But you should keep the same directory structure. If you modify something inside the admin folder, then you should rebuild the admin after that or you can run the app with --watch-admin flag, which will detect automatically changes to the admin part and will autobuild it with autoreload.
https://strapi.io/documentation/developer-docs/latest/cli/CLI.html#strapi-develop