I uploaded several file to cover all the municipalities, provincies and countees of Catalunya, because it was the only format I found, that the public authority use to share this contents. is it ok to later select a spefic city perimeter?

Great that you have found and added those files! Catalunya is going to be well represented :)
From what I've seen there are these three files:

1. Locations of municipalities, I've renamed it and removed the NUTS3 boundaries tag. Seem to be the centres of each municipality. It is actually also not around city boundaries, but rather a point on a map.
2. Municipalities of Catalunya 2. These are the city / municipality boundaries for all cities. Looks good.
3. Municipalities of Catalunya 3
   I don't think that these are really the NUTS3 boundaries. In Spain, NUTS3 are the provincias, see here, so Barcelona would be the province in your case. Are these LAU boundaries perhaps? Looks like the comarcas (counties), no?

For Vallès Occidental, you can later on select the boundary from File 3. For cities in Vallès Occidental, you can select them from File 2, although for now, let's try to stick to one dashboard :)

Yes Carolin, you are right regarding File 3, those comarcas are like counties, like a suBsystem of provinces (NUTS3).