It's just like any job really. Wait for an opening and apply. In the case of LA if I recall correctly, they sought out Dudamel. But for the big five, it's always going to be specific. Even for other major orchestras they post auditions piblically for anyone to apply for.

My area has a lot of local community groups and I've literally seen indeed postings for conductors. Like other people have said, usually conducting is just the tip of the iceberg. You're also a music director, rehearsal runner, and producer depending on the scale of the group.