You’re right; I don’t think it goes over it!
The charms we currently have are called Polished Pet Charms, and they can be farmed through WQs in Battle for Azeroth, Shadowlands, & Dragonflight. If you have mission tables unlocked, I believe the Shadowlands tables still have them, but I cannot speak for the Battle for Azeroth one.
In BfA they introduced level scaling, so you can battle the enemies with < 25 pets, and the enemy teams will scale to the highest level pet on your team. This applies for BfA, SLs, & DF. It’s a good way to build up charms if you have a lot of lower level pets.
Keep in mind, the website I linked is going to have strategies for teams of 25, so it’ll be a good source once you build up your roster, especially when you want to get into the more difficult battles like Family achievements and pet dungeons.
Anywho, you can use the charms to buy the instant 25 stones, the purple ones called Ultimate Battle Training Stones for 35 Polished Pet Charms.
The Marked Flawless Battle-Stones, or blue ones that will get your pet instantly to rare quality, are 15 PPCs.
A stack of 25 bandages are 5 PPC & Flawless Battle-Training Stones (a flat xp gain for your pet) are 3 PPCs each.
The three expansions vary in how you unlock their WQs. BfA includes getting to max level for the xpac (I’m pretty sure) and going through a some of the campaign chain with Magni, then unlocking them at your capital, I believe. It’s been a while on that one.
For Shadowlands, you’ll need to get to max level, and there should be a breadcrumb that will pop up sending you through unlocking them.
Dragonflight is going to be the easiest. You get to max level, unlock them, and then all your characters that can access the Dragon Isles will have the WQs unlocked. The downside to DF for pet battle WQs is that we only have 1 until 10.0.7.