For example: Why would I trust a site that is selling supplements to tell me how much of which supplement I should have?
I know that if something is free, you are the product, and so I'm willing to pay for memberships or subscriptions as well as free resources
I would suggest finding a local Crossfit box (gym)[0] or USA Weightlifting Club[1]. Crossfit gets a lot of hate but it is very effective for developing all around cardiovascular capability, strength, mobility, and fitness provided you do it right. (Olympic) Weightlifting will also develop strength and mobility but there isn't as much or any focus on cardio conditioning. Gymnastics is very effective but there aren't many gymnastics clubs catering to adults.
You will be exposed to proper nutrition, recovery, training concepts, and community at good Crossfit boxes and USAW clubs.
[1]https://www.teamusa.org/usa-weightlifting/club-wso/find-a-cl...
> Why would I trust a site that is selling supplements to tell me how much of which supplement I should have?
In the end it's just a source of income for an otherwise difficult business (as you're selling knowledge - which is easy to duplicate).
In the end this is not rocket science. If you follow a variety of people you'll see they converge on the same insights.
Even if they don't, it's good to expose yourself to variety of approaches and pick whatever works best for you.
And last but not least - you don't have to know everything from day one! Fitness, like many other things is best approached under a bootstrapping lens. Just get it going and build up from there.
https://squatuniversity.com/ - articles and videos related to proper form and mobility.
The site and YouTube channel are a wealth of information. The "hypertrophy made simple" video series is probably what you want to start with.