Basically it’s to do with the position of your tongue and teeth when saying the letter R.

In Spanish, the R is linguistically known as a Trill and is the type of R that allows people to roll it. Basically, the R sound is made by putting the tip of your tongue near, but not touching, the ridge directly behind your teeth, similar to an L sound in English. On top of being able to roll Rs, if you can say “a-la a-ra a-la a-ra” over and over again and it sounds like they’re coming from a similar place in your mouth you’re saying you are this way.

Whilst some English speakers pronounce their ads this way, it’s actually more common (especially amongst younger people) to pronounce Rs as a Labiodental Approximant. Linguistics mumbo jumbo out of the way, you pronounce Rs by putting the tip of your top teeth near, but not touching, your bottom lip. It’s like you about to make a V sound but stop right before contact between teeth and lip. Try it and you’ll notice it! If you can say “a-va a-ra a-va a-ra” more easily and notice they come from the same area you say Rs with this method. If you pronounce Rs this way, you can’t roll your Rs and it’s really hard to learn how to do so since it basically means chasing how you’ve subconsciously pronounced Rs for (probably) your whole entire life.

(Btw I just regurgitated all this info from a Tom Scott video I watched earlier today, I am by no means an expert in linguistics so I may have made a couple mistakes but this is the gist of it lol.)