First of all, you shouldn't follow video guides. They become outdated quickly, and often leave out important details. Also, all flashcarts have microsd slots. Microsd cards are about 10 dollars for an 8gb, and usually come with an adapter so you can use it in your 3ds too.So, something I noticed when looking up video tutorials on how to CFW a system.
Flash carts have MicroSD slots on them, which works fine for the New3DS and new 2DS.
Which flash cart do you use if you’re on an Old 3DS that has only a normal-size SD card?