I was curious how staffing in classical orchestras work and below is a good explanation from Quora (about the US).
https://www.quora.com/How-does-seating-and-hiring-firing-in-...
My former teacher (he's been a concertmaster of a major city symphony for years, and is also well versed in the different ways in which one can ask "Did you actually practice?") wrote this explanation:
Think of it as management and labor. Management is Executive Director, Music Director, Principal Guest, staff, etc. Labor is orchestra musicians. Then you have the Board, which is there to raise money. Then you have the volunteers. Bear in mind, orchestras are non-profit, 501(c)(3).
So between management and musicians there is a CBA, or collective bargaining agreement. Just like the Major League Baseball has one between players and owners. In the CBA, you'll find chapters on hiring and firing. Each Symphony has its own CBA.
Usually, hiring is done by auditions behind a screen. If you're hired, your first year is the probationary period (sometimes two years). At the end of it, if management deems you fit for the job, you are granted tenure, just like at a university when you're applying for a teaching position. You don't need to re-audition, unless it is for a different title (from section to title chair, etc.), in which case you do. Firing, if it must come down to it, is also explained in the CBA.
The big symphonies are part of AFM (American Federation of Musicians). Then you have regional orchestras, community orchestras, etc. Some might be unionized, some not, but essentially it sort of works the same way.
Some concertmasters, like in England, are contracted outside the CBA, so they may be on a 3 or 4 yr contract, or more. But I think it the US, most are part of the CBA.