Since most people aren't going to have the setup for using an Interchangeable Lens camera just to do webcam stuff, my serious answer is that even an old phone is a better webcam than about 90% of webcams, mostly because the phone will handle object tracking and noise isolation better than your Logitech or Microsoft whatever.
DroidCam allows your phone to be used this way and yes
there's a version for incorrect devices as well.
That being said, my partner occasionally has professional need for quality streaming and I use it as well. We have the gear because we both do photo and video work aside from streaming, so we tend to have nice stuff.
Main camera is currently a beaten-to-death Canon RP with an 18-55mm RF-S lens on it. It basically doesn't get moved or messed with and even using the RP in 10MP crop mode makes excellent 1080p video. The second camera, and the one I'd suggest to normal people who don't play computer games on Twitch, is an
ObsBot Tiny 2, which has good optics and AI subject tracking since it includes gimbal controls. This is somewhat expensive for a webcam but it is way, WAY better than everything else we tried, although we use it in 1080p rather than 4k.
Since I use the same setup to run tabletop games and occasionally for IT training course, I have pair of old action cams as well. One of them lives on a fixed overhead mount and the other one is just on a long HDMI cable that I can use for close-ups of objects.
The whole thing is run into a 4 port HDMI capture card and run through OBS studio. You can get away with multi-input streaming over USB 3 devices so long as you have enough root hubs to connect all of them to, but then you can get in to weird corner cases where moving a mouse can cause dropped frames if it's connected to the same root hub as a capture card.
For audio, I either use a Shure MV7 or the Rode wireless Lavs. Both of these options are around $150/mic but
you will be judged for having bad audio a lot more than you'll be judged for any other part of your setup. A 240p webcam and decent audio > a 4k video stream + hearing every bit of environment noise and/or having to yell to be heard.