If you're planning to work on the codebase, you'll find this useful!
The entry point is in index.ts. You can have the Slack server and the GRPC server running at the same time when NODE_ENV=development, or you can set NODE_ENV=production and SLACK_BOT=true|false to run either service. The documentation is in /docs and is a Next.js static website that uses @hackclub/theme.
The codebase uses Prisma as the ORM for connecting to a Postgres database. The schema can be found in prisma/schema.prisma, and extra database utilities (including extra types) can be found in db.ts.
All of the Slack routes can be found in lib/slack. lib/slack/slack.ts contains the actual Slack initialization and an extra middleman function, execute, that catches errors and has logic for permissions and new users. There are some extra views in lib/slack/views.ts, such as the loading dialog when modals are loading. More info can be found by looking through the Slack API documentation.
The GRPC routes can be found in /lib/api; they're all imported by router.ts, which in turn is imported by the entry point. These routes also use a extra middleman function, execute, that does the same thing as the Slack middleman function of the same name. These routes are based on proto/bag.proto. The GRPC client can be found at /client. Every time you update proto/bag.proto, you need to run yarn run rpc:gen to regenerate the actual .ts files describing the layout of these routes. More info can be found at https://grpc.io. You'll also need to run yarn run build in /client in order to re-transpile the client for it to be up-to-date with proto/bag.proto.
There are a few extra YAML files. api_config.yaml and gcp_update.sh is for deploying the GRPC service to Google's Cloud Platform, so you can ignore it. blacklist.yaml is a list of channels, by name, that the Slack bot should not be allowed to run in. buf.gen.yaml is actually the configuration file used to generate output files from the GRPC proto file. maintainers.yaml contains a list of maintainers, and manifest.yaml is the Slack manifest file for the bot that you should replace with your own.
A few files that haven't been covered are generally self-explanatory and/or aren't needed for the codebase to be run locally.
If you have questions, ask in #bag-dev in the Slack!