I started training around 14 years old, running and calisthenics. At 17 I switched to an athletically focused high school. I have gone deep into rabbit holes on periodization and other programming. Mostly, I appreciate how simple the principles are. Do more of thing you want better, it get better, for specific training. For general fitness, it is quite similar: Find some movement you at least don’t mind, and maybe even enjoy, and do it regularly.
And also the ‘tism likes to see number go up.
Simple is not at all easy, mind. And many peoples general fitness levels would be greatly improved by some more specific training exercises. Some pulling and pushing with the upper body, and some hip mobility, maybe even loaded.
This is an interest I share with my eldest son. And he seems to enjoy it for much the same reasons: Effort put in according to a plan gives predictable and consistent results. And we can both agree that many internet fit-fluencers are full of shit and need to stop overcomplicating things. To illustrate:
What is the optimal split? If optimizing for hypertrophy (not everyone does), probably 2-3 times/week for the same muscle group. But really, whichever makes it easier to be consistent. If that means every day for an hour, or once to twice a week where it can fit in, so be it. If going very frequently, be aware of rotating focus on consecutive days. If less frequently, stick to a program for a while, and keep intensity and/or volume high.
Until my current sick leave period, I made great progress training one to three times a week.