Bottle Caps are a useful currency in Pokémon Sword and Shield because they allow you to Hyper train one of your level 100 Pokémon. You can find them by several methods, such as buying them from a Battle Point (BP) dealer at the Battle Tower, viewing them. after beating Max Level Raids, through Digging Duo and Competitive Ranked Matches. Sometimes, you may get lucky and discover a gold bottle cap.
Gold bottle caps are different from traditional bottle caps because you can use them with Hyper Trainer to maximize all of a Pokémon's individual values (IVs), rather than a single one. To do this, however, you must have a level 100 Pokémon. The Hyper Trainer is in the Battle Tower, on the right side. Talk to them if you have a regular or gold bottle cap to make your Pokémon much stronger.
Getting gold bottle caps isn't an easy task, though. You will find it hard to spot them. You can find them through the conventional means of regular bottle caps, but you have the odds stacked against you.
You cannot buy them from the BP vendor in the Battle Tower. You have to reach a high enough level in the Battle Tower challenge to receive one for a reward, and it's a high level, so you'll need to practice a lot before you find a gold bottle cap in your inventory.
The other method is through the Digging Duo, located in the Bridge Field area of the wilderness, near the Pokémon nursery. She talks to one of the brothers to spend 500 watts to get them to start digging. They will dig up treasures and you will receive numerous different prizes. Bottle caps are rare, so the chances of finding a gold bottle cap with this method are slim.
You may have a chance to spot a Gold Bottle Cap from Pokémon Max Level Raids, but you'll likely only see them through the five-star choices. If you can get some help from responsible players, these five-star raids shouldn't prove to be a problem.
Reliably, you'll have to take on the Battle Tower challenge to see how far you go. The higher you go, the more chances you have to add gold bottle caps to your inventory.