First of all there are many, many, caminos. I have road biked three of them, the coastal camino from Bilbao, camino Frances from just north of the Pyrenees and the Silver route from Seville.
Ryanair fly to Seville and to Biarritz for camino Frances. Aer Lingus fly to Bilbao.
Most bikers stay in Hostels, Pensions and a few camp. Cyclists sometimes have to wait till 7 at night to get beds in pilgrim Hostels, especially in high season.
If not camping one should get away with less than 6kg of baggage.
One sees all sorts of bikes,hybrids, mountain and racers.
The airlines will carry your bike, but it must be prepared for flight.
Aer lingus fly back from Santiago De Compostella.
Bikes can also be hired, Tour and Ride are based in Santiago and will deliver to your chosen starting point in Spain. Bike Iberia also rent bikes.
Michelin do a special Santiago Map, its good showing the camino trail and the nearby roads. for details use google
Google " Confraternity of St James London" for further info.
Spain has a great motorway network , As a result N roads nearby often have very light traffic. Spannish drivers are respectful of cyclists.
I have had 6 bike trips to Spain , I think its the best