SSS, CSS, CR, slope, PCC... what the hell does it all mean?

I took up golf in 2021. I see all these acronyms on scorecards etc and while I can easily google them, it isn't clear to me which are actually a "thing" any more.

My understanding is as follows:

CR - the score a scratch golfer would be expected to score, on average, around a given course.
SSS - the same thing? Not used any more in WHS?
CSS - an adjusted version of the above for a given competition day. Also not used any more?
Slope - a factor used to calculate playing handicaps under WHS, based on the difference of perceived difficulty between a scratch golfer and a bogey golfer (around 20 HCI for a male, I believe). Was this a thing before WHS?
PCC - competition handicap correction. Has this replaced CSS? Used in conjunction?

Also, what does par actually mean for a golf course? Does it just come from the course length? Because to the layman it sounds like the score you should aim to go around in... but isn't that CR?

The Par is the value per hole, and overall for the course, in which the course is "designed" to be played. So, during design, a par 3 hole, the expectation is that a "typical" golfer is able to get on the green in one, a par 4 in 2 and a par 5 in 3, followed by 2 putts. I use typical golfer loosely, because as I will go on to discuss, holes can be vastly different, where in relation to par they can be very easy to extremely tough.

A par 4 might be 290 yards, with some bigger hitters able to get there in one, and most having a short shot in for 2. Or, it could be 450 yards, with many not able to get there in 2 at all. So, if you had 18 300 yard par 4s, and compare that to 18 450 yard par 4s, there would be no way you could just say you'd expect a golfer (say a scratch golfer) to go round in 72 shots just as easily on either course. That is where Course Rating comes in, and that tells us what the expectation would be (not the average score, but a score playing at their handicap level) for a scratch golfer. SSS was the same thing, although CR is to 1 decimal place, SSS was a whole number.

There is also a Bogey Rating, which does the same thing for a golfer with a handicap around 18-20ish. From these 2 values, a Slope is calculated, and with that the relative difficulty of the course can be accounted for between low and high handicappers. Some courses high handicappers will need more shots than the better player, others not so many. Slope did not exist pre WHS in the UK, although it was used elsewhere, such as the US.

PCC effectively replaced CSS. Worked out differently, the calculation is a secret, so many have been dubious about the outcome of what PCC ends up as each day.