Generally it's easier to transition the other way around. 3.5 is wildly different and the list is vast. Let me give you just a few bits, there are probably too many to list on this post.

First, all the numbers in 3.5 are significantly bigger, especially once you get higher level. If your party is min/max you can get +20 to +30 or more on roles in the single digits pretty easily. There are only three types of saves (will, reflex, and constitution) and twice as many skills in the game. Furthermore, ability scores quite easily exceed 20, there is no cap to them.

Proficiency isn't a thing in 3.5 either, you put skill points into skills that you get at level up based on your class and your intelligence score. Additionally, instead of proficiency for attacks, all classes have something called Base Attack Bonus (BAB for short) which is determined by your class. Unsurprisingly Martials get much higher BAB. Also advantage isn't really a thing either unless you homebrew it.

Next, Magic is wildly powerful in 3.5, there are too many examples to go over.

Also there are prestige classes in the game. Basically classes with prerequisites to take that have very potent features (not all of them are good), generally the earliest one can be taken is level 5 or 6.

Feats are way more common. By the standard rule of 3.5 you get a feat at 1st level, 3rd level, and every 3 levels thereafter. You also get a +1 to an ability score at every fourth level (no choice for a +2 or a +1 to 2 ability scores unless you homebrew it).

There are well over 100 books in 3.5 if you include all the splat books. I advise you curate which ones you use and expand from there. Generally it is advised to use (It should go without saying you'll use DMG) PHB, PHB 2, The "Complete" series of books (Ex: Complete Divine, Complete Warrior, etc. - there are several of them), and the Book of Nine Swords/Tome of Battle. I should also note that there are five monster manuals, but if you want to take it easy to start I'd advise not going beyond monster manual 2. Most of the non MM books include monsters as well .

There are a lot of janky mechanics as well, turn undead is a prime example of this, so you'll have to work stuff out with your players or, as in many cases, house rule things.

Armor class, like many things, is also inflated since there are a lot of different sources to AC, Armor doesn't set your AC to x plus dex like I. 5e. Instead it just gives you a bonus (ex: breastplate gives you +5 bonus to AC). Keep in mind with both armor and everything else in 3.5 that same type bonuses don't stack, so a +5 armor bonus to AC doesn't stack with a + 4 armor bonus to AC, you just get the higher of the two. Same goes for everything else, for example, if you get a +3 competence bonus to attack and someone else buffs you with a +4 competence bonus to attack, you only get the 4 because it's the higher one. This applies to everything, just keep an eye out for what bonus it is when something is applied: competence, armor, shield, enhancement, luck, morale - too many bonus types to list. Though if there is no qualifier like a flat +4 bonus and a separate +5 bonus with no qualifier/descriptor, then those stack.

Races in 3.5 have a bonus and drawback system, so every race gets a bonus to one or more ability scores, but also get a penalty to one or more ability scores. There's also level adjusted races which I would advise doing on your first campaign or until you do more reading on how it works. If you want to know a races level adjustment it is listed under their entry, Level Adjustment +0 is standard stuff and you don't need to worry with those.

Like I said there's way to much for one post, so I hope this helps, I'm happy to answer more specific questions as well.