In this animation, the top area displays the tiles as they're stored in memory, and the bottom shows what's displayed on-screen. Unfortunately, this particular technique won't work in SmileBASIC, because BG data doesn't wrap around (if you scroll off the right edge of the BG, you don't end up on the left side), but something similar can still be done.
That said, this is definitely more complicated to pull off than loading the entire map in one go, and depending on how you do it, you may need to make tools that support the way you store your map data.
Games on tile-based consoles (NES, SNES, GB, etc.) solved this problem by loading in new map data as you scrolled. For example, normally the NES's video chip only has enough memory for a two screen wide/tall map, but games like Super Mario Bros. and Metroid were able to have maps that appear much larger with this technique, by not actually storing it all in memory at once.Umm actually, you can imitate wrapping around by putting a bit of the beginning at the end and then when the player passes that you use bgofs to move it to the beginning and imitate that. But yes, you can just have it load a bit then another when you move on.
