When he came in, Southgate (along with Dan Ashworth) definitely changed the approach of the national team from picking either "the best" players, or "the form" players, and treated it more like a club where there was a settled core of players who would work across multiple international periods to learn the patterns of play and style that Southgate wants. My personal opinion is that Southgate and his coaching team, e.g. Steve Holland, over-achieved from low expectations in Russia 2018, but since expectations have been higher, there have been missed opportunities, e.g. the Euros final Vs Italy.
The reason this is important for the Premier League is it's not about whether player X or Y is in form - if they haven't been in the setup before, they are unlikely to make a squad and even less likely to play.
This is of course just an outside observation - I don't know Southgate's selection policy, but it was pretty well documented that is the structure he and Ashworth wanted to have. It gives much greater stability and allows for more complex tactical structure, but the majority downside is it takes away a lot of flexibility and definitely makes it seem like he has his favourites who are undroppable regardless of form (or in Maguire's case in the past... actually playing at all)
