Qatar hosted world cup and they started from scratch building stadiums setting up everything and after World cup they dismantled some of them and made sports centres to remaining. Qatar is country who's not developed in terms of football I'm comparison to America Europe who have a developed system . In fifa 2026 many stadiums are already build so they don't have to start from scratch .
I feel like even the surroundings will not need too many things. Sure there could be some revamp on design of some of the hotels and such but that would be mainly regarding decorative stuff and not the whole buildings, whereas Qatar had all of that made, from hospitals to even schools and many other things, and of course roads and infrastructure which costs a lot to build.
Stadiums weren't the biggest cost, that was less than 10% of the entire cost, it was bringing all those people together, building hotels, building roads, revamping the airport and many other things that combine together made it so costly. ALL of that is available in these three nations and they will not have to build anything from scratch to make it happen.