GET a Web API method with query parameters (read methods: conversations.history, auth.test, …).
GET a file URL (url_private_download) with the token; resolves the raw response.
Open (or reuse) the token's DM with userId (conversations.open); resolves the channel id.
POST a JSON body to a Web API method.
Post text to a channel, DM or thread (chat.postMessage); resolves the message's channel and ts.
Upload and share a file: files.getUploadURLExternal, POST the bytes
(an HTTP error throws "Slack file upload failed: HTTP status for
filename"), files.completeUploadExternal. Resolves the file id.
The Slack Web API calls core and instances use.