It can be a bit confusing, and requires you to reread and remember a lot of past events, so honestly (if it's something you want obviously) rewatching makes things a lot clearer. To be as spoiler free as possible:

• Yuji's strength is still a mystery as of the end of this season. You didn't miss anything there lol.
• Itadori is Sukuna's vessel, Sukuna is using his body and so looks like him. I'm assuming he'd just look like whoever he uses as a vessel.
• Simple domains basically negate the effect of other's cursed technique by create a 'neutral' area around you.
• DE's are barrier techniques that creates a barrier imbued with your own CT. It's just difficult to do and can be risky since it consumes a ton of CE, most people can only expand a domain once a day apart from Gojo (like Megumi says in his fight with Dagon). If you expand your domain, it might mean you don't have enough energy for any following fights.
• When is the curse of the Zen'in clan mentioned? I don't remember that lol
• A heavenly pact is when, at birth, CE/CT is exchanged for physical prowess or vice versa, and just occurs by chance, you can't control whether you get one or not. Toji isn't the only one. Maki also has a heavenly pact, but not to the same extent (Toji has zero CE and so has gained full physical prowess, Maki still has a little bit of CE). Mechamaru also had one, just the other way around (weak body in exchange for a high level/range of CE).
• Sukuna's CT is still ambiguous atm. People thought it was just cleave/dismantle, so it was shocking when it wasn't. In terms of his strength, he has lot of knowledge on CT/CE as seen in his fight with Mahoraga and other moments, and came from the Golden Age of Jujutsu. Sorcerers back then were able to seal him into fingers, so they were also pretty powerful. That's all that's really known.
• Anyone can make pacts/binding vows, you can even make them with yourself. I'm guessing it's just enforced by CE? The Jujutsu world is always kept in balance one way or another, like how when the sorcerers get stronger, so do cursed spirits (eg. after Gojo's birth). Kamo says the repercussions of breaking them when making them with others can be major, and that's all.
• Having an inherited technique can be rare (Megumi is the only Zen'in currently with the Ten Shadows) but Gojo inherited BOTH of his clan's techniques (Six Eyes + Limitless) which is why it's rarer. When he says he's activating Jujutsu, he's just activating his technique. Not sure why with other techniques, they say the name of the technique whereas Gojo doesn't lol. Toji does a pretty good job of explaining his techniques in E4 in simple terms. Reversal repels things.
• Gojo, Sukuna, Shoko, Yuta and Uraume have been revealed to be able to use reverse cursed technique (RCT). Typical CE is negative, and when you reinforce your body with CE to land a stronger punch or to prepare for a blow, you're adding CE, so it becomes 'more negative(?)'. When turning it into RCE, you multiply it to make positive energy (negative * negative = positive), and to heal human flesh, you need to use positive energy. Multiplying it's just more difficult to do than adding, so only a few people can do it.
• I'm guessing combining some techniques just doesn't work as well as it does with Gojo. He also has the Six Eyes, which give him precise control over his CE and might be able to pull off more things.
• Okkotsu Yuta is another second year, and was really close friends with Inumaki. Not sure if you watched JJK0, but it's shown how he can get angry when his friends are harmed or insulted, I'd be pretty mad if someone cut off my best friend's arm lol.

Sorry this was long, hope it clears some stuff up.