How would I teleport to a map uisng a specific filename, ie teleport to SHOPS.dmm, and when i choose the x,y,z the Z is only for that file and not for other maps

Ie the files (amount of maps)

Overworld.dmm (75)
Shops.dmm (40)

teleport(Shops.dmi,x,y,1)//teleport to the first map in Shops.dmi

//the code here
I don't know if lummox's swap maps can do that, and im not sure if byond can even do that, but im sure, theres an easier way of changing maps.

put that anywhere on the shop map

src.loc=locate(1,1,1)//or where ever you wan to go

is the code to change maps.
You can really only have one map in a game at one time. If you include multiple map files, they just get added together on compile (on additional z-levels). If your first map file has 3 z levels, your 2nd map file will end up starting at z #4, etc.

A coordinate-independent solution is often more convenient, such as using locate() with conjunction with the tag var, or a type path. Note the latter isn't reliable as-is if you have multiple turfs of that type.