why is: a girl en flicka and not ett flicka?
It just is. Flicka is a n-word. Most living things are n-words...but as usual there are exceptions.
Ah OK so it isn't a feminine/masculine thing?
it's a common and neuter thing
This is so hard to pronounce properly (the struggle is real).
The sound of 'och' here is pretty bad. Usually people pronounce it like 'ock' or 'ogg' with a somewhat quite ck/gg.
I would say it's actually pretty common to pronounce 'och' just like 'o' like she does here, but I guess that might differ between regions