How come some Jalapenos are basically mild/hot and
some are almost inedible they're so hot?

I haven’t noticed much difference between jalapeños.. I can tell you we make jalapeño poppers about 2-3 times a week and if you clean the membrane and seeds from inside my wife and kids can eat them. If you don’t take the membrane and seeds I can barely eat them and I love hot stuff.

Want some hot ones put them next to some cayennes. They will cross be hot as heck.

I bought some whole ones in a jar to eat with Santitas.
After slicing one up I licked my finger and lit me up!!!
And better wash your hands afterwards, do not get that
stuff in your eyes.
These are so hot even the smallest little piece is all I need,
and I like hot stuff.

I’ve been told that the only hybridization occurs in the seed the following year

This year I planted bananas and jalapeños and my bananas are 2x as hot as my jalapeños. Unsure of variety however

My jalapenos this year are so hot I can barely eat them with the seeds/membrane removed. I'm not sure what kind of variety I got but they'll make you sweat even just nibbling on it.

Also some advice from a friend, even after washing your hands be careful going to the bathroom after dealing with them hot things. Couple weeks ago he made us some poppers and a few hours later we heard a full grown man scream from the bathroom. Gave us all quite a chuckle at his expense.

I don't know about the hybridization but the ones that were right next to he cayennes were way hotter than the ones toward the end of the row. They made some killer Cowboy Crack though!

Lot of different hybrids out that have varying degrees of heat. I always plant some TAM jalapenos that originated at Texas A&M. They're like the football team of their namesake...kinda lukewarm.