That means I can rename the nvidia .txt file nvidiabittube for example and do a batch file like this below?
xmr-stak -c nvidiabittube
And the same with the pool and url ect. and it won't generate a new file too? thx

And in that same folder where xmr-stak is I can add another coin too?
Yeah.
But you better also add .txt there if your config is a text file, or it seems to create a new config "file" without .txt for me if I dont do that. [unless you want that]
xmr-stak -C poolX.txt
NOTE: Notice the capital -C and non-capital -c in the launch parameters: -c, --config FILE common miner configuration file
-C, --poolconf FILE pool configuration file
I recommend using --poolconf and --config instead, to avoid confusion/mistakes in the future.
xmr-stak --poolconf poolX.txt
Similar for nvidia config:
xmr-stak --nvidia nvidiaX.txt
or you can combine them:
xmr-stak --poolconf poolX.txt --nvidia nvidiaX.txt
or add more:
xmr-stak --poolconf poolX.txt --noAMD --noCPU --nvidia nvidiaX.txt
There are also some "pre-launch" variables you can play around with:
For example my .bat file for CPU+AMD mining with some GPU variables set I found in some guides.
[ didnt really notice much effect though, might be different on nvidia, or might not work at all

]
setx GPU_FORCE_64BIT_PTR 1
setx GPU_MAX_HEAP_SIZE 100
setx GPU_MAX_USE_SYNC_OBJECTS 1
setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_MAX_SINGLE_ALLOC_PERCENT 100
xmr-stak --noNVIDIA --cpu CPUFull.txt
The "setx" might only apply for Windows by the way. [its different for different OS]