That matches what I've been told by various personal trainers. 6-8 reps if focusing on strength, ~12 for all round, and 16-18 for size/endurance. Do three sets, weight should be enough that the last couple of reps on the first set are a bit of a struggle. Subsequent sets just push through as far as you can.

Your trainers clearly never read Starting Strength.

No idea, I certainly haven't. This was decades ago, though, so it's entirely possible that established best practice has changed.

these days your better of not reading that, probably. bunch of outdated and bad advice coming from that corner.

What do you recommend?

"Eat food, not too much, mostly plants."

Also probably move around a lot, doesn't matter how, ideally by finding something fun to do that involves moving around a lot.

This is a common myth that came out of nowhere and has been debunked.

1-5 reps for strength. 5-30 reps for hypertrophy.

This article claims that's false, that 8-12 at higher weight leads to the same result as 20+ at lower weights.

The research is studying young untrained men. Everyone puts on muscle at mach chicken when untrained.