How can I add add-ons to my game on the Nintendo Switch?

Yes, you can add addons to minecraft bedrock on switch, the thing is, it's a bit complicated, and I don't have a switch so I don't know much about them, but here's a tutorial someone made:

https://gbatemp.net/threads/how-to-install-custom-maps-texture-packs-and-addons-for-minecraft-bedrock-for-nintendo-switch.542742/

The tutorial explains how you would add addons to your minecraft game, it essentially tells you to use your switch SD card, insterit it into your computer and then add the addons there, then adding the SD card back into the switch now with the added addons.