I'm trying to get a level 1 Hisuian Growlithe but after I thought about it I wasn't sure I could since you can't breed in PLA and you can't trade Hisuian Pokémon to SWSH or BDSP to do so but I also can't get a clear answer anywhere.
The lowest level possible in the game code can be assumed to be level 1, like previous Pokémon games. The lowest level Pokémon you can find in the wild, however, is level 2 (Aspiration Hill, Obsidian Fieldlands). source: experience catching pokémon If you get a level one pokémon in a trade, it is therefore either transferred from a previous game (SwSh/BDSP, etc), or hacked (if it's a Hisuian Pokémon, such as said Hisuian Growlithe you mentioned). Hope this is helpful :)