In addition to what others have said, if FGO is your priority, it’s worth looking into iPhones - but only up to the iPhone 8. Newer iPhones have hardware-level compatibility issues with FGO’s outdated version of Unity Engine on NA, causing the game to lag like shit despite the more than sufficient hardware to run it perfectly smoothly.
We’re likely looking at a September 2021 release for the engine update on NA, if we follow JP - and following JP is likely in this case since an engine update is a huge undertaking, and the version pipeline of updates to FGO are likely to make it either impossible or an unreasonable amount of work to push it early to NA. Until said engine update, iPhones load the game magnitudes faster than Android does (we’re talking 70-90% faster load times here), which is a considerable convenience imo.
That said, if FGO isn’t your only priority, getting a recent Android device is obviously the better choice, as buying an iPhone 8 in 2020 seems like a pretty bad investment, unless you intend to upgrade again in less than 2 years. If you don’t mind the longer load times until the engine update lands, the game should run smoothly on any recent at least mid tier Android phone.
Personally I’m using a Honor View 20 (Kirin 980 SoC) and the game is essentially lag-free save for very few graphically intense NP animations (Columbus is the only example I can think of; even Gil’s NP which is notorious for lag runs smoothly on this phone), and a small stutter when plugsuiting servants.
If you want more specific recommendations, you’d need to tell us your budget range first. But as I said, as long as it’s a fairly recent phone, even mid tier phones should run the game super smoothly, so there’s no need to go for high-end if you wanna save some money.