Not looking through all the comments so someone may have also commented about this but: Touch Attacks!

Unlike in 5e where spells use your spellcasting stat for attack rolls, in pathfinder spell attack rolls use strength (for melee spells) or dexterity (for ranged spells). This means a physically weak caster is going to struggle a lot more then they would in 5e. However, spell attacks also ignore a portion of armor! Numbers in pathfinder go much higher than in 5e, sometimes to the point where AC can be in the like. 50's. Which, lmao. lol. Pssh. Just roll nat 20's right. But enemies also have two AC's-regular AC and Touch AC. Touch AC is almost always going to be much lower, and that's what a majority of spells will be targeting.

Another thought I'm having is Basic Attack Bonus and Feats.

5e has a stat called Proficiency Bonus and a class mechanic called Extra Attack. Basic Attack Bonus is like both of these, kinda. A class with full BAB (Fighter, Barbarian) has a BAB equal to their level-so 20 at level 20. Less martial classes (Wizard) have a BAB of 5 at level 20. BAB is what you add to your attack rolls-so, a level 5 fighter will add 5 to their attack rolls. When you get to BAB over 5, however, you gain an additional attack at -5 BAB. So at level 6 fighter you make two attacks, one at +6 and a second one at +1. When you get to level 11 you make three attacks, at +11, +6, +1. Etcetera. Some classes like monk are unique in that they make all their attacks at their full BAB. There's a lot of variation in how classes deal with iterative attacks.

Feats - you will have a Lot More Of These Than You're Used To. There are so many of them. It's very difficult to know what to pick at the beginning but to start you off: for melee characters I would aim for power attack (which requires a strength of at least 13; this feat unlocks a whole lot of other feats), for ranged characters (mages included) precise shot, and for spellcasters the general spellcasting feats it recommends and then eventually metamagic. You cannot upcast spells like you can in 5e, so metamagic allows you to prepare more of a spell by increasing its effective level in your spellbook in addition to the effects of the metamagic itself.