Unions take about 2.5% of your income and give you a tie-tac to wear on your tie. Most of the airline jobs are union and you have no say in joining. You join or quit the airline. Hopefully, you get good pilot volunteers to work for you and represent you. I got greedy pilots who volunteered for the union and who steal from the working pilots. We did get rid of them, so hopefully things will get better. Airline unions do not have the power they used to.

Bidding: There are two types of bids. 1. Monthly line bidding: you bid a "line" of flying or a schedule of flights on your current airplane. It is in seniority order, so the senior guys get the best flying, money, and days off.
2. System bidding: you bid the aircraft that you want to fly and the position in that aircraft: 737 first officer or captain, 777 first officer or captain, etc. It is also in seniority order, so the big planes (with big paychecks) and the Captain positions go senior. 777 Captain would be senior while 737 first officer would be junior. You bid what you want and get what your seniority can get you. Your seniority is when you were hired. All pilots hired before you are senior to you according to their "date-of-hire", while all the pilots hired after you are junior in the same way.