Almost everything you see for sale or for rent in Catalonia is online, which is both the blessing and the trap. The portals give you the whole market on one screen; they also give you the whole market on the same screen as everyone else, and the houses people actually fight over often never appear there at all.
Idealista and Fotocasa are the two names that matter in Spain, and together they show you the shape of the market: which streets in Alella carry a sea view, what a six-bedroom detached house with a pool costs in Cabrils against Teià, and how fast asking prices have moved over the last two years. Use them to build your mental model of price and product, even if you never buy a house you first saw there.
There is also a practical point. A large share of what appears on a portal is already sold, reserved or priced for negotiation. The listing is doing two jobs at once: finding a buyer, and giving the agency a reason to call you and ask what else you might be looking for.
Off-market, or a pocket listing, is a house the owner has decided to sell or let without a public advertisement. Usually for one of three reasons: they value discretion, the property has an issue they would rather discuss face to face, or they simply do not want a queue of strangers at the gate. At the luxury end in Maresme this is common. The best houses in Llavaneres, Cabrils or Sant Vicenç are often shown to a handful of people before a single photograph is taken.
There is no off-market portal. Anyone who claims to hold "the database" is selling you something. What actually exists is a network: the local agents who know which owners are thinking about moving, and who pass those houses between themselves before anything is public. Access comes from being known inside that network, and that takes a little effort.
Off-market does not mean unchecked. Get your own valuation, because with no public listing the seller controls the narrative on price. Check charges and ownership at the Registro de la Propiedad, not just through the agent's summary. Ask why the property is not being advertised; sometimes the answer is discretion, sometimes it is a problem you would rather know about now. And keep an independent lawyer in the loop from the first offer, not from the notary's desk.
The honest summary is that the two routes are not rivals. Portals tell you what the market is worth; the local network tells you what is actually available. Families who get the right house usually do both, in that order, and start talking to agents a good while before they are ready to sign anything.