Incoming webhooks
Create a secret address for a channel and anything that can send a webhook can post there. Slack-compatible messages work as they are, and GitHub's own events (pushes, pull requests, issues, comments and releases) arrive as readable summaries, checked against a signing secret.
Workflows without code
Post a message or create a task when someone joins a channel, on the days and time you choose, or when a message mentions certain words.
Custom slash commands
Add your own /command that calls your system, signed with a secret so you know it came from your workspace. Your system can answer just the person who typed it, or the whole channel.
A REST API
Personal access tokens, read-only or read and write, let scripts read channels, messages and tasks, search, post messages and create tasks.