I don't really think it's that mysterious! Obviously we are not privy to all the of the factors but it is still pretty simple. Alvarado gets the highest-leverage spot with the best match-ups for him (which will generally include lefties) and so does Hoffman (generally including righties). Doesn't matter if it is the 6th, 7th, 8th, or 9th, though the first two innings are still more likely to go to Kerkering or Strahm. Soto's saves have been in games where Alvarado wasn't available or had already pitched, I believe. We've seen Strahm close in the past too and obviously they want Kerkering to be able to as well (Seranthony also but right now he is not in that high-leverage group).
And the Phillies have to be winning or tied in all of the above too of course. On Thursday Soto pitched the 7th in a tie game only because Strahm had already pitched and it was the bottom of the order. Pretty sure it would have been Alvarado otherwise even though that could have put Soto into a higher-leverage or extra-inning situation. Then it was supposed to be Hoffman/Alvarado 8/9 but Hoffman blew it, which got Ruiz into a game where they trailed.
It is a far better way to run a bullpen, and they obviously have a system that lets the pitchers know and prepare for their spots, even if it's not a specific inning. It's also why they probably won't trade for an established closer; or if they do, he'll be expected to buy in.