Here is my memory from playing a bunch of Palladium games, including TMNT, in the 80s and 90s.
Combat is at base like AD&D with add ons. Roll a d20 roll high to hit. I think anything above a 5 hits, but armor provides a higher target, if you hit above that armor target you hit them, if not the armor takes the damage (but has a certain amount of SDC (structural damage capacity) that it can take then it stops protecting you. If they hit there are active defenses you can take depending on your combat skills, dodge, parry, and roll with punches. Each defense is a d20 roll and if you beat the attack the defense kicks in with dodge and parry avoiding damage, roll with punch reducing bludgeoning by half I believe. You have hit points which is your physical endurance attribute (constitution equivalent) plus 1d6 per level. In everything but Palladium Fantasy 1e you also have SDC that are a type of hit points used first.
Character creation is involved. There are normal D&D like stats to start with some bonuses for high scores. There are Occupational Character Classes that give a framework of stuff and some choices. I can't remember if TMNT has that or a bunch of random backgrounds with more freeform choices.
There are combat styles like ninjitsu which depending on specific ones give different bonuses at different levels, some combat styles are more attack focused with attack bonuses and extra attacks, some are more defense focused with things like auto parry (unlimited parrying in a round) and defense maneuver bonuses, a lot are not balanced against each other.
There are some things you take that give bonuses in weird places like stats and SDC or specific combat maneuvers, so taking running increases endurance or something like that. Special notice to boxing which among other increases gives you a flat extra melee attack even when using other fighting styles.
Then there are skills which are percentiles with a small random advancement per level (something like a d6 increase per level) and a short description of what they cover but no real description of how to use them in actual play.
Different backgrounds in TMNT give different base stuff and choices of skill type things to choose, a more educated person can take a lot of stuff and build themselves up a lot depending on what they take, less education focused ones actually get short changed a bit in that they can't take as many buffing little extra stacking things like boxing and running.
TMNT also has a minigame of mutant animal creation building where you can basically be a tradeoff of a spectrum of human to animal features built from a human baseline, so point building whether you are almost entirely human looking with hands and bipedal and human size, or saving some points on being a little smaller or non human looks or semi or non usable hand or bipedal stance to use those points to get animal features or even psionic powers or spending points to be bigger. So you could be small bird looking person with flight capable wings (the sample crow hero), or little bears with terrifying psionics (sample villains). It breaks down a bit if you start as a big animal, if you want to be a mutant elephant the spectrum is big with no animal features other than looks and no human features, down to human baseline, down to small with some elephant and human features. Features allowable are determined by the specific animal base, so no mixing elephant tusk natural weapons with bird wings on the same character.
Advancement is like AD&D xp advancement charts but instead of xp for money or defeating monsters there is a chart of things that give specific xp awards like clever ideas in play.
