Check out rootmetrics for a map of coverage in different areas.
In terms of 5G coverage - I think it’s a pretty safe bet to say that all 4 networks will be rolling that out over the next few years and it’s just a case of where your area is in the priority list for how quickly you’ll be getting it.
However, for a ‘standard’ smartphone 5G doesn’t really bring you that much benefit other than getting ookla speed test screenshoots to impress your friends with. 5G benefits are really about things like being able to cut your cable/landline connection and run your whole household via 5G but even when 5G is rolled out fully, that’s unlikely to happen with the data caps on mobile connections.