Bikes 🚲

ok, so i'm really trying to understand zone 2 training. hypothetical - this isn't how i train, not looking for training tips, just trying to understand the difference. 
let's say I ride religiously 1x week for 3 hours. in study 1, I ride fully in zone 2 in study 2, I ride in zone 3. both are on a trainer with the same settings. after 6 months of this - what's the difference between these two?
i feel like i have an intuitive understanding that does not align with what I believe I've heard/read.

I know you're asking for real assistance but I asked chatGPT because I wanted to know too. I feel like sometimes I struggle to maintain "zone 2" because based on my heart rate I'm "zone 3". Maybe I need to test my max HR. 

I feel like eventually some Zone 3 becomes Zone 2 because your body increased efficiency and it's easier y'know