I’m almost 40. Tore achilles 6 years ago. Injured right hip squatting without warming up hip flexors 2 years ago. I am performing physical therapy exercises 3x a week and seeing improvements.

I have met people that only stairmaster, bicycle, other crap (think sled pushing type stuff) that have bigger legs than people squatting 500+.

However

9 times out of 10 i got the impression it was genetics + being fat as a kid or lifestyle related ie growing up on a farm. I’m very worried about pushing my body to the point of needing hip replacement or achilles surgery so i’m wanting to weigh my options.

no of course you can't, what the hell kind of question is this?

why would a fit persons' body ever need to build on muscle in the legs just from walking and riding a bike?

I'd say it's most likely genetics + how much you trained growing up, type "muscle memory"

Are you able to do machine exercises? My legs were biggest (like constantly ripping the crotch of jeans, big) when I was doing the following Tuesdays/Friday.

Squats 3x5
Stiff Leg Deadlifts 3x8
Hack Squat Machine 3x12
Leg Press 3x12
Leg Extensions 3x12
Hamstring curls3x12
2 calve exercises 3x12

long story short - a **** ton of volume. On Fridays I'd do sets of 15 and sometimes add dumbbell lunges, and the last 4 exercises would usually supersetted together (quad set).

Keep in mind this was about 5 years ago when I was only working part time and going to university. It was a lot easier to focus on the gym. Working full time now I feel like I'd die if I tried to do that much volume after an 8 hour day lol, but I feel like you could cut out the squats/deadlifts if they're too tough and go very high volume machine work.

Ofc if you cycle around town hard enough

I bet if your legs are built to be able to hit the stair machine all day and at any time.

I ride an airbike and it gives me crazy pumps in my legs but that thing has heavy resistance.

IDK that just riding a bicycle will necessarily result in big legs.

Same way years ago I realized it's pretty hard for me to both run often, and keep mass on my calves at the same time.

Sure if you add resistance somehow.. weights, low gear on bike, etc.