You just find books with the solutions.

Some like Hubbard and Hubbard's Vector Calculus come with a solutions manual.

Some like Knuth's Concrete Mathematics contain full solutions.

There's also whole genre of books like Schaum's Problem books and their Outlines which contain thousands of solved problems. And they're quite cheap.

For any given math subject X, you can probably search for an "X problem book".

That's mostly because proofs are too long to write out easily in the back of a textbook, imo. Your best bet here is to learn the proofs of the theorems proved in the book first - they're usually much harder to come up with at first glance than the problems one bases on them. Or, if you're using a commonly used textbook, you can usually find people's own proofs online. Sourcing an easy to grok and easy to check proof is a very good skill to build in any case, it minimizes wheel reinvention at the neural level.

If you have a textbook you're interested in studying from, you can usually find courses from searching on Google that use that text. If you're lucky, they will make the homework and answers available online.

If you search long enough and look for older courses (before all of these online learning platforms like Canvas became popular), you can usually find lots of worked examples. There is no central location to find what you're looking for, but you should be able to find supporting resources.

Read the book and whenever you come to a theorem, stop and try to prove it on your own. If you can't, read some of the proof, then try to continue on your own.

Hop on the mathematics discord and ask in their book channel. It's the big server with a picture of a torus as the icon. There are some very knowledgeable people there