It's exactly the same as Ronaldo, just that he played a few more games. Amad was bought for the first team, not the U18 or U23 (like Hannibal). He is a first-team player in every sense of it, and a regular for his country. U23 has played three games, Amad hasn't been involved in any. So no, he won't play much with the U23. The point is that Elanga and Amad are fighting for a first-team spot, along with James and Mata for example. They are a part of the squad and train regularly with the first team, and not with the U23. And no, United lists players as first-teamers that are part of the first-team and might, in a few cases, play with the U23. D'Mani Mellor is listed as a U23 player, because he is one. Amad is 19 and listed as a first-team player, because he is one. Simple as that.
It's not the same as Ronaldo in the slightest, and only someone with the loosest grasp of reality would say it is.
Again, Ronaldo had played over 30 games for Sporting when we signed him. He was a first team player for Sporting and he was signed to be a first team player for us.
Amad had played in just 5 games for Atalanta, all of which were 10 minute cameos off the bench at the end of games. He was signed as a future prospect, albeit an expensive one, and a prospect he remains.
Amad is slightly different to the others you listed, I'll give you that, but if he's not loaned out, he'll almost certainly float between the U23s and the senior side. At most we'll see him in some cup games and maybe against the cannon fodder at the bottom of the PL as the season develops. The reason he's not featured anywhere yet is due to his involvement in the Olympics, which is also the reason Bailly hasn't featured in a squad yet.
As for him being a "regular" for his country, he's made 3 appearances, all off the bench, only one of which was in a competitive fixture where he played the last 5 minutes.
Elanga, Shoretire, Hannibal and Mengi are all in the EFL Trophy squad as U21s. You don't get much more not-first-team than that.
If you want to count any player listed on the club's website under "first-team" as a first team player, by all means do. I just can't for the life of me see why you'd get yourself so wound up about the size of the squad when a) clubs competing on multiple fronts will be very likely to have 30ish players feature in some capacity across the season (some of which will be young players making one or two appearances), b) a good chunk of the players you're crying about won't even have to be registered because they're under the age thresholds, and c) as referenced, they aren't first team players and will only feature very sparingly, if at all.
The fact you're now comparing Amad and Elanga to Rooney and Ronaldo says it all, really.