Pokemon Sword and Shield Type Null is a weird one. You can't catch it in the wild like you do most Pokemon in the game; instead, you're given it as a gift, much like how to get Charmander in Pokemon Sword and Shield. You can only get Type: Null in Pokemon Sword and Shield from one specific place though, and the synthetic Pokemon is certainly unique thanks to how it can change type depending on the item it holds. Here's how you get Pokemon Sword and Shield Type Null.
How to get Type: Null in Pokemon Sword and Shield
Put simply, you can't get Type Null in Pokemon Sword and Shield until you've completed the game. Yeah, unfortunately the legendary Pokemon is gated behind completion of the main story. When you do become champion however — follow our complete Pokemon Sword and Shield walkthrough to help get there — go back up to the Rose Tower, which is now the Battle Tower.
Enter through the main door and take a left to find a woman stood with Type: Null in the open. Speak with her and she'll give you Type: Null for free, just like that! Who'd have thought that you just need to become the strongest trainer in the world to get one of the most mysterious legendaries?
How to change Type: Null's type in Pokemon Sword and Shield
Type: Null is unique in the sense that it can change type to be any type in the game. When you speak to the lady who gives you Type: Null, she'll also give you a series of "memories". These are items that you can make Type: Null hold depending on what type you want it to be. So while you can't switch type mid-battle, you can swap the type after each battle.
Unfortunately, you can't breed Type: Null so you're stuck to just having one for your playthrough. It does evolve into Silvally when it has high enough friendship though, which you can check by visiting the first house on the right in Hammerlocke and speaking to the old woman. She'll also give you the Soothe Bell, which can be used to increase friendship.