That’s more of an hour long conversation than a short answer
Key factors
-Food
-Varying intensity for both bjj and gym
-Sleep
-No kids
-Live alone
-Making them both priorities
I train when I’m tired, I train when I’m injured, I train most days of the month. HOW that is done is more complicated.
I’m certainly not doing a high volume leg days to failure and running off to 2 hours of grappling.
If you listen to wrestlers talk about their sport, US wrestlers were all horsepower and endurance, but Russian wrestlers were more technical. US wrestlers were injured more frequently. Why? US wrestlers trained at max all the time, Russians would use less intensity and more repetition. That’s not saying Russians weren’t elite with their condition, or that US wrestlers weren’t technical... but each country sort of learned the benefits of each other’s methods, eventually marrying the two as time went on.
Long story short, it’s the how, when, and why of training. More isn’t better. Harder isn’t better. Better is better.