Bottle caps are a useful currency in Pokémon Sword and Shield because they allow you to train with as many of your level 100 Pokémon as possible. You can find them in a number of ways, such as buying them from a supplier of Battle Point (BP) located in the Battle Tower, see them after defeating Max Level Raids, via Duo Digging and Ranked Ranked Matches. Sometimes you might get lucky and discover a Gold Capsule.
Gold Capsule differs from traditional bottle caps, as you can use them with Hyper Trainer to maximize all of a Pokémon's individual (Pokémon) values, rather than just one. You must, however, have a level 100 Pokémon to do this. The Hyper Trainer is in the Battle Tower on the right. Talk to them if you have a golden Gold Capsule to make your Pokémon even stronger.
Getting Gold Capsule isn't an easy task, however. You will find yourself in a difficult situation to locate them. You can find them through the conventional means of using the classic caps, but you have every chance to do so.
You cannot buy them from the BP vendor at the Battle Tower. You must achieve a rank high enough in the Battle Tower challenge to receive one in exchange for a reward. It is therefore a high ranking. You will therefore have to practice thoroughly before finding a Gold Capsule in your inventory.
The other method is to use the Digging duo, located in the Bridge Field area of the Wild Area, near the Pokémon Nursery. Talk to either The Brothers to spend 500 Watts and start digging. They will look for The treasures and you will receive many rewards. Bottle caps are rare, so the chances of finding a Gold Capsule with this method are slim.
You might have a chance to locate a Gold Capsule during The Pokémon Max Level raids, but you will likely only see them among the five stars. If you can get assistance from a responsible group of players, these five-star raids shouldn't be a problem.
You will surely want to take the battle tower challenge to see how far you go. The higher you go, the more likely you are to add Gold Capsule to your inventory.