Along with one-trip tickets, there is a variety of transport cards to suit your different needs to travel on Bizkaia’s public transport network.
If you are visiting Bizkaia for a few days, you may prefer the Bilbao Bizkaia Card. It is a tourist card that provides unlimited use of most public transport and valid for 24, 48 or 72 hours, along with offering other advantages. http://www.bilbaobizkaiacard.com/
There is the Barik card, accepted on all public transport in Bizkaia, for longer periods or when travelling in groups of up to ten people. https://www.ctb.eus/en/como-funciona-barik