Of course it is, why wouldn't it? Girona is a real city with 100,000 souls that work and live there... It's always "open", LOL!
The little towns in Costa Brava are the same thing, people live in each one of them so while there might be some more summer-oriented businesses (ie. certain cafes or hotels) that might be closed off-season -so to speak- the rest of the village/town continues life as usual. Take, for example, Cadaqués (3,000 permanent residents), l'Escala (10,000), or Roses (22,000), among many others. Yes, compared to the peak of July, when they're jammed with visitors, in March, they might feel "emptier", of course, but that's a great thing so you can visit them properly without crowds. Everything else is open all year round: the museums, the Natural Park, the monasteries, etc. My thought, anyway.
Furthermore, many of them have an additional floating population that regularly resides in Barcelona but maintains apartments/houses in the town for the weekends.
You mention tour guides... In Catalonia, you can very easily DIY almost everything; you don't need to "be taken to". When you have your rough plan and have done your initial research, simply ask for the details here.
Catalan: No, there'll be no offence if you attempt to communicate in broken Spanish, we understand you're here for a few days -moving over for the long term that'd be a different matter ;)
Some links you might find useful:
https://costabrava.org/en/
https://www.girona.cat/turisme/cat/index.php
https://www.tarragonaturisme.cat/en
