Because they have these things called SDKs to extend their product, for specific programing languages.

Here is an example, https://www.sanity.io/studio

Want to extend the SaaS product for your backoffice users?

Learn React or make Studio from scratch.

Frontend stacks are also backend stacks in cloud products.

Because with Next.js, Vercel was able to turn the frontend stack into also a really shitty backend stack. And it's particularly shitty at being deployed, so they're in the business of doing that for you.