For martial arts Black Widow trained under the Russians to be a killer, Gamora was forced to fight her adopted sister under Thanos for the same reasons as BW, and while not MCU Nolan’s Batman trained with the League of Shadows. I don’t think Captain America was really an “expert” in his first appearance but in later movies it’s not unreasonable to think he trained a bit with Black Widow while at SHEILD considering how close they seem to be in WS. Spider-Man is always shown practicing his swinging except in the MCU to save on showing his origin once again. His enhanced senses likely help with that. Hawkeye, yeah. We don’t really get anything and it’s mostly assumed how he trained
You just outlined the story elements of 'how' they got to be.. while not really addressing the degree or credibility of what they achieve in said amount of time.
Iron Man - he gets a battery pack, and tech.. and somehow he can turn into this amazing fighter, stuntman, etc. It takes more than one scene of wobbly flying before making it credible that they can now outfly virtually anything.
Capt America goes from wimp to pumped - and somehow evolves into this super fighter in no time.. including amazing shield throwing skills
Loki is more a mage - yet somehow has all these h2h fighting/combat skills
Ant Man - goes from zero to hero by just picking up tech
killmonger - just accept it.. we don't even bother
every demi-god in the universe - hela, thor, etc..
red skull - again.. somehow energy/power gives you everything else you've been lacking in life...
Peter Parker is still a kid - yes enhanced senses, strength - but all this agility, movement, everything else.. again it's the usual 'comic book powers make you into a bad- trope' that permuates almost every comic book story.
People want to bust on something like Rye or Danvers... Peter Parker should be exhibit A!
Detective I mean... who solves and decodes all these 'masterminds plans'? It's always our heros in the story. And what makes them qualified to do so?? Uhh.. they saw some stuff and they are the hero of course!
And I never said Carol was a Mary Sue if you thought I did.
And I say it's a ridiculous argument to have in a comic universe story to even begin with. Comics by their very nature were always skimping on the 'build up' and history. They explicitly use extremes to enhance the idea of danger or uniqueness. 'extreme' and 'impossible' are nearly synonymous with comic book super hero stories.
Notice virtually every villian is just somehow... evil and advanced. Do we really know why or how? No.. Why is it every villain can somehow create a standoff in a fight against our 'best of the world' heroes at least to some degree? Because if they just got wiped out without any struggle it would be a quick and boring story.
And lets not forget these superheros largely are supposed to exist in OUR universe with our baseline of understandings. They make heroes unique by building up their abilities. Not justifying them or qualifying why they deserve to be that level.
We always get a story about the hero for 'how' they got there, but it rarely justifies the outcome.. it just gives us something for history and progression for the character.. and to give ourselves some ideas of what their powers should be able to do. We need some structure for our brains to accept this character as believable in some sense.
Where characters like Rey stick out is because we were conditioned by every element of the story before that made it seem like these Jedi were super special.. it takes tons of training and sacrafice to harness these things.. you need people guiding you.. and even then, our main story people are supposed to be the creame of the crop. So we build up all these 'rules of the universe' so to speak in how things operate. Now, here comes a character that breaks virtually EVERY ONE of those rules and stands toe to toe with what are supposed to be the creame of the crop. That's why she stands out so bad. She's a conflict in her own story universe.
That's not really the case here with superhero comic stories.. because its generally the INVERSE of complexity or challenges to get where they are. That's why its pretty lame to even throw that label around here in these stories. IMHO
Maybe it’s a sign 
