Terraza etiquette?

Depends where you’re at and how busy the restaurant or bar is. Oftentimes you can seat yourself but on many occasions, especially if busy, someone will seat you. If it isn’t busy the waiter may just make a gesture to seat yourself. Waiters at bar terraces will occasionally glance over to see how things are going and will sometimes ask if you need anything, but mostly they’ll leave you alone. With restaurants it’s more like the states where the waiter will stop by, but not as often. In Europe they try not to be intrusive. When you want the bill you generally need to get the waiter’s attention, gesture or state you want the bill, and eventually they will bring it to you. Be patient if it appears you’re being ignored, you’re not, they just take their time.

Yes you have to be proactive. You can do this by making eye contact with the server and raising your hand slightly in the same motion.
They know the tourist routine. There is usually a host directing things.

There are typically 4 kinds of restaurants (not including fast food) in Spain:

Bar-Restaurante - these low-key establishments are frequented most by locals during the weekdays. They usually serve a 'Menú del día', an excellent seasonal three course lunch. Here one usually sits one's self on the outside sitting unless there is a reserved placard on the table. The waiter then comes over to see what you want if just drinks or food as well. The waiters will check in on you from time to time, but can be considered slow compared to American standards.

Restaurantes - more upscale affairs with table cloths, wine menus etc.. Here there is typically someone you talk to first to get seated (like a host in the USA), they usually will be standing near the entrance. This is most similar to the US in terms of service.

Tapas bars - these places are usually located in the more touristy centres of cities and will have long bar tables with different tapas on top, here you usually make your way to the bar first and talk to the bar tender directly to get service or if you want a check, waiters will not come to you here, you have to be more proactive. In Granada however is different, you usually order the drinks first and then they give you tapas for free with the drink orders, you can always order larger raciones or higher quality tapas separately.

Gastronomic food markets - these are where local restaurants have food stalls located in the main market, that sell street food to eat on site. These newer types of food markets have become quite popular in Spain in recent years, especially for those looking for a quick drink and a bite to eat. You just order what you want at the counter then sit down at any empty table then pick up the food order when its ready and eat at your table. An example would be the Mercado de San Miguel in Madrid.