I can`t talk much about Ultrecht but its blasphemy suggesting he didn`t build a team at Ajax. If your logic is correct Poch didn`t "build" Spurs.
Lloris, Walker, Rose, Vertonghen, Kane, Winks and Eriksen are the starting 11 players he found there thats more than half the team, could probably add Lamela. Credit where it`s due developing Kane and to some extent Winks but everyone else was proven at a decent or high level so he didn`t really build them rather inherited a strong foundation he built on using your logic.
I`ve done some digging looking up Ajax`s team from the EL Final in 2017 and Onana is the ONLY player currently that was part of that team who`s still at Ajax whereas Ziyech, De Ligt, De Jong, Van De Beek, Veltman, Schone, Dolberg and Neres are players he found at Ajax and either improved or promoted from the Academy and he added Tagliafico, Blind and Tadic.
That`s a number of starting 11 players he may have inherited but between 2019-2021 De Ligt, Veltman, De Jong, Schone, Van De Beek, Ziyech, Dolberg and recently Neres have all been sold and he`s replaced them with Lisandro Martinez, promoted Mazaroui/Dest(Dest got sold too) promoted Gravenberch, Edson Alvarez, Haller, Antony, Klaassen and Berghuis. In addition he`s recently promoted Timber who`s one of the most promising CBs in the world. As you can see yes you`re right he did inherit a lot of players that lead to his good run in 18/19(as did Poch) however a lot of those players have been sold in a short space of time and replaced mostly with players of lower quality and Ajax are still punching above their weight. Don`t question Ten Hag`s team building ability.
In that period ETH has had to rebuild Ajax Poch ONLY lost Walker prior to 2019 who was replaced by Trippier(signed a season earlier) and Aurier. He kept the rest of his players and added Alderweireld, Wanyama, Dier, Sissoko, Alli and Son to his starting 11 discounting the "squad players" he signed and while he did a solid job in the PL it`s disappointing he didn`t win an FA Cup or Carling Cup as he had a genuinely top squad.
Pochettino in the one and a half seasons he was at Southampton never lost any key players but added Osvaldo, Chambers, Lovern and Wanyama in addition to Lallana, Clyne, Shaw, Ward Prowse, Gaston Ramirez, Gazzaniga, Fonte, Schneiderlin, Lambert and Jay Rodriguez amongst his other key players who were already there. Yes he did a solid job improving and promoting the players there but if you compare and contrast Pochettino and ETH`s team building stints the latter wins this one hands on since the former never faced the challenge of losing a lot of key players from the existing core he inherited and having to replace them in a short space of time.If he ever did that and STILL maintained the same levels yes you could argue Poch`s team building is better.
Ten Hag on the other hand has never had the luck of keeping his key players for a short space of time but he`s still managed to do a similar impressive if not more impressive job than Poch. Most smart clubs would gamble with the man who`s shown he can do it regardless of the team he has with the possibility of hitting elite levels with his own team over a man who`s had the luck of keeping his players and recently managed a super team but still fallen short in one way or another.
