I'll chime in on the discussion of super-high ability scores and a PC stealing most of the spotlight:
It doesn't happen much in my experience. Having one or even two 20s will give a PC an edge, no doubt, but the game is too complex and too swingy with the d20 for that to account for any sort of long running, consistantly felt edge, with the SOLE exception possibly being a CON 20.
CON 20 can make things feel very off-balance with other PCs have CONs 14 or 12. In one game with had a Barbarian with STR and CON 20... so great offense numbers of course but also stellar durability! Or other martial, a Paladin, had STR 16 and CHA 20 IIRC, so was decent at offense (great when he nova'd!) and had a great AC due to plate armor, but his durability with CON 14 just couldn't match the Barbarian. Consequently, he felt like the proverbal glass-cannon to the player.
Meanwhile, the DM could never get the barbarian down! To make the issue worse, he even took the Tough feat so by the end of the game (level 12), he had something like 180 hp and good AC around 19 IIRC. Meanwhile, the Paladin had about 100 hp or a bit more. So, even though he had a better AC (22 I think?), he just couldn't tank enough to front-line like the Barbarian--especially when you add in the damage resistance due to rage!
Overall, their abilities were about the same average, because the barbarian was low-average in most of the stuff, but two 20's; while the Paladin had generally higher scores, but just the CHA 20 at the end.
So, despite the barbarian having STR 20 and the paladin STR 16, offensively they were well matched due to smites. But that second 20 in CON just made the barbarian impossible to bring down.
Other party members felt it, too, because creatures would down them more easily, while the barbarian was still up in the end.
CON is the only real score where I've seen this sort of issue come up, personally.
I'll take your word for it. I don't think I have ever seen a PC start with a 20 Constitution in 5E. I've seen them start with an 18 using non-RAW methods of rolling that don't allow you to move abilities, but usually they are putting bonuses elsewhere and putting their best score elsewhere.
Generally when I am playing point buy I am going with a 10 or 12 Constitution, occasionally I will run an 8 with either a Dwarf or the Tough Feat. The only time I ever run higher than a 12 on point buy is if I have something that keys off of it, like Rune Knight or Damphir.
I am an oddity though, most other Players do make it one of their higher stats, often the second best. For me, I find putting a lot of emphasis in Constitution at the expense of other abilities makes you weaker in the social and exploration pillars. I also don't play Barbarians FWIW.