How to Get and Use Bottle Caps in Pokémon Sword and Shield
Bottle caps are an easily overlooked item you'll encounter in Pokémon Sword and Shield. They go into your Treasures category in your bag, which many defaults to a great way to make money fast. However, they have a new use, and that's maximizing an Individual Value (IV) of one of your level 100 Pokémon.
You have several options available to you to capture these useful items. There are bottle caps available in the Battle Tower for Battle Points (BPs), and they cost 25 BPs each. You can find the vendor at the Battle Tower off to the left side in the middle. It takes time to acquire 25 BPs, so don't rush the process.
Another straightforward method is to provide the Digging Duo, located in the Wild Area next to the nursery in Bridge Field with 500 Watts. Those 500 Watts will have one of the duo start digging, and they're going to have a chance to pull out a Bottle Cap. There are other things they could pull out, too, so you have to do this method a few times before you have a bottle cap show it.
Players have reported when they have a high-level Pokémon with the Pickup ability has found a bottle cap during their travels. It's not a bad idea to run around with a single Pokémon with the Pickup ability who is nearly level 100 to try this, but it won't always happen.
After you've beaten the game, another method to acquire bottle caps is completing Max Raid battles or in competitive Ranked battles. Both are viable methods, but you want to have the best Pokémon available to go down these paths.
When you have several bottle caps, return to the Battle Tower and speak to the vendors on the right side of the building. They're willing to take a level 100 Pokémon through Hyper Training, but only if they're level 100. When they come back, the Pokémon should have a maximized IV.