Idunno much about dsp in general. All I know is that he's one of the first streamers ever, he's bad at videogames, he doesn't like it when people make fun of him/critisize him, he's a shit cook, and occasionally ebegs but I never understood what he did to merit such extreme notoriety across the Internet.
He just seems like a normal guy. Maybe a little thin skinned but the extent with which people go to fuck with him seems completely disproportionate and is bound to make anyone irritable.
At the risk of sounding overly pedantic, he wasn't one of the first streamers ever, he was actually pretty resistant to the idea of streaming when it first came around because he thought it was a passing fad. Which to give him some credit, wasn't necessarily a bad thought at the time, but he dug his heels in for way too long and to this day he still doesn't really get streaming versus content creation. He was one of the first big YouTubers though during the wild west days where pointing your camera at a TV was still considered novel and you could easily strike gold with ad revenue and spamming the algorithm, which, hey, intentional or not, deserves some props.
His ebegging is way more than an occasional thing and is pretty consistent during each stream.
Phil is about as far from normal as can possibly be. There's a huge list of Phil mannerisms that are outright alien, but they all basically stem from the fact that the dude has 0 real social interaction and only leaves his house one day a week to go shopping and sometimes get his hair cut.
Phil is incredibly thin skinned, and despite what he likes to insist, nobody really fucks with him despite his residence and phone number being known for many, many years.
Not trying to gatekeep Phil or anything, there's a lot of Phil lore to know. I think Phil is sort of unique as, unlike with other lolcows, there's not really one defining incident you can point to that makes Phil a cow. With someone like Keffals or Boogie or Bossman, they tend to have like, one or two major defining moments that made them a cow. But even Phil's most infamous moment, jerking off on stream, isn't really what made him a lolcow and is honestly one of his more "innocent" moments in the grand scheme of things. Like, yeah, it's embarrassing, but that makes him a punchline more than it does a cow. With Phil, it's more of a "death by a thousand cuts" thing. Individually, if you go through his lolcow moments they don't seem particularly thread worthy, but then when you see how long the list is. When I was first getting into Phil, I basically lurked the main thread for a few weeks and started catching on to why he was thread worthy/how he had such a long thread to begin with. And then more events get referenced on top of the current stuff and eventually you just pick up on most of his weird habits.
If you want more of an abridged version of why Phil is interesting
https://kiwifarms.st/threads/what-is-the-appeal-of-dsp.181190/ is an alright thread to skim. There's a little bit of initial slapfighting, but otherwise there's a few glimpses into why everyone thinks DSP is a cow.
Oh, and as to the last point, I don't think Phil actually has a lot of notoriety these days. IMO, he's kinda become a cult-cow. He was more known back in the day, but to use bad movies as a comparison, it's kinda like how most people these days will know something like The Room or Birdemic versus Manos or Plan 9. To most people, he's just that dude who jerked off online once and plays video games poorly, but that only touches on the weirdness that is Phil Burnell.