The UK Anti-doping agency performs about 2000 tests per season on Premier League players, and every player must have an Athlete Biological Passport. How do you suppose they have all managed to avoid this?
COYB!
John.
The thing is also that a very specific method has to be developed to detect doping compounds. And in the compunds themselves you don't have to change a lot to make them undetectable for these methods. In some cases just a adding a methyl group to an anabolic steroid makes it undetectable for assays that would have caught the unaltered steroid.
Also, they have a lot of clever people who can set up a nice testosterone cycle that significantly boosts performance without reaching threshold levels for detection.
A very famous case for this is FC Bayern's Leon Goretzka:
On the left is Leon in May 2019. On the right is Leon in May 2020. The transformation took place during a one-month (!) Covid break. Here's a trained athlete who just out of nowhere puts on ginormous amounts of bulk in his mid twenties in an insanely short time period. No positive doping test turns up. They are always one or two steps ahead.
Edit: Of course, other Bayern players bulked up as well. They won the Champion's League that season...