The integration center is recipe-based, the same way monday.com's internal automations are: pick a trigger from one app, pick an action in another, and it runs on standard fields. The Slack connector posts a message when a status changes. The Salesforce connector can create or update a lead when an item moves. The Gmail and Outlook connectors turn an incoming email into a board item. QuickBooks, HubSpot, and Mailchimp connectors exist too, though they cover a narrower set of triggers than the bigger-name integrations do.
Webhooks fill the gap for anything not on the list: monday.com can call out to a URL when something happens on a board, and can receive an inbound webhook to create or update an item. That's the same mechanism we use when we build a custom connection monday.com's marketplace doesn't ship natively.
If you're after the full GraphQL API that powers all of this, including authentication, rate limits, and what's left to the apps framework, we cover that in more depth on our monday CRM API page rather than repeating it here.