All skills are calculated against their base skill xp.
You can't see skill xp, but it's been more or less confirmed that it's there.
To best explain what gets calculated and at what rates, I think magic level does the best job because we can see exactly how much mana equals each level.
There's a chart here.
https://tibia.fandom.com/wiki/Magic_Level_Table
Those numbers apparently aren't exactly and get less exact in the higher levels, but it should still act as a good guide for answering this question.
Basically, the other skills are calculated the same way to various degrees only it's hits given/taken instead of mana used.
The exact numbers and exact what anyone's skill experience is at is a bit of a mystery. But we do have good guesses out there to find like the one I linked.