What does it feel like to squish on someone? I've heard that when you have a crush on someone, you'll feel "achy"; when you are with a person you have a crush on, you feel all your attention being sucked towards them; you prefer to interact with a person

To me, a crush is a desire to have a romantic relationship with someone, or to at least to fantasize about doing such. You might fantasize about taking them on dates, holding their hand, kissing them, etc. Not to say that the desire to perform these actions isn’t sometimes present in squishes, but it’s definitely more present in a crush.

In my experience, a squish is more like “wow! That person’s really cool! I wanna be their friend/BFF/QPP!”. You might fantasize more about, say, watching movies with them, or talking with them about a subject you both really like, or things that generally are more associated with friendship than with romance. The farthest I’ve ever gone with a squish is the desire to hold hands and to maybe live together in a platonic roommate situation.

I wouldn’t say that things like jealousy, an achy feeling, or wanting to impress someone are necessarily crush-exclusive. I’ve definitely felt all of those when I’ve had squishes in the past; it’s just that the context is different. I might be jealous over say, seeing someone I have a squish on hang out with their best friend, rather than seeing them with a romantic partner. 

And honestly, just like crushes, squishes can range anywhere from “I think you’re cool and wanna hang out!” up to “PLEASE BE MY BEST FRIEND OMG I LOVE YOU”. It’s just that the relationship desired, and the actions desired, are different.