It depends on their risk assessment based on all sorts of stuff (perhaps like your IP, browser info, mouse movements etc.). If you're suspicious (VPN, Tor, heuristics etc.), they require the phone confirmation.

Can confirm, I can create a google account on android without phone number but I cannot do so on desktop. When using the android-created account on desktop, it requires (not optional) a phone number.

A phone number on a fully functional google account isn't optional, at least in western Europe (though I doubt another country is better because then people would just VPN to there).

I once did some work for Google that required accounts to do the project (existing accounts weren't a good fit for the project). They couldn't provide us with accounts because it was a huge hassle and another team, so I ended up standing in line for a long time getting my identity verified a half dozen times for various prepaid sim cards with throwaway phone numbers and I think we billed them for it. If there had been another way, they'd simply have told us...