The AP was 100% correct on their 2020 calls. They have 4000 reporters on the ground checking in with vote counts, and they use the math of how many votes are left and where and previous voting tendencies for things like mail-in votes or overall political leanings (like say, if DC actually got their own representative, they’re not going to flip from 90% Democrat to 51% Republican in 2 years… that’s extreme, but that’s the idea). They call it when it’s very unlikely to change.