I've done plenty of reading on gaming sites and insiders, so for the best I can do:
Microsoft: They really needed to expand their first party with the increased competition and the paltry offerings this gen, which now puts them around level with Sony in terms of number of studios. The thing is, some of them came out of left-field (Playground Games is like Insomniac in terms of the relationship, but at no point would I have expected Ninja Theory and Double Fine Productions to be acquired), so I have no fucking clue.
Amazon: They're in the game, but I don't know what they're doing.
Sony: Since their CEO Jim Ryan already talked about acquisitions to investors, and we've just seen Insomniac join their first party, I expect more. Remedy has been rumored. I know they've been in talks, but I don't think anything is final, and the talks were early last I heard from a few Reset insiders a few months ago. It's possible they want to see how Control is received and how it does. Kojima Productions is another that I'm sure is at least thought about at Sony, but an issue with that which someone on another board mentioned, is that if you buy a studio, it's great to buy a studio with a culture in it where the wheels keep turning no matter who's at the helm (like Naughty Dog). Kojima Productions revolves around Kojima, and many of those employees likely joined it for him. Whenever he retires, are people guaranteed to stay? Dunno. So there's a risk to that.
Bluepoint has a long relationship with them and have great development chops. They've done a good job remastering/remaking older games in Sony's catalog, and they're working on a big project right now (don't know with whom). Housemarque is a studio that has admitted their next game likely won't perform well sales-wise. They're working on a AAA game currently, but I know their arcade games were not bigger sellers with Sony, which is why they're shifting away from Resogun/Alienation/Matterfall/Nex Machina stuff. So I don't necessarily know how they fit, but considering Media Molecule and PixelOpus exist, I think they'd be able to fill a multi-player/pick-up-and-play niche in the whole lineup. Plus, I don't know how well they're doing financially, and I'd hate to lose them if their AAA game doesn't pan out.
There's also Ready At Dawn, which I feel would have been part of it already had it not been for The Order: 1886's underwhelming sales and reception. I still think they're really talented from their other games, especially when you see their VR success with Lone Echo on Oculus Rift, so if you want to expand your studios and also beef up PSVR (Insomniac has shown they can do that to), it seems like it might be a reasonable possibility considering their past relationship with them. Supermassive has a long history with them as well and honestly did the whole Quantic Dream thing better than QD with Until Dawn. They worked on VR games, Playlink games and standard PS4 games with them, so they're versatile, but right now, they seem tied up on The Dark Pictures with Bandai Namco, so I dunno.
That's all the obvious stuff. There are a bunch of smaller independent developers who have worked with Sony on one-off projects, but it does seem as if they acquire studios after having worked with them as they did with Bend, Naughty Dog, Insomniac, Guerrilla, Media Molecule, and Sucker Punch, and the ones who no longer exist (Incog, Zipper, Evolution).