I can tell you how flows work at Envoy.

You interview with Envoy and get hired. Congratulations, you have now also been hired at AA. When your name comes up in seniority order you select YES or NO. If you select YES, you get a class date. Done.

Envoy gets 50% of each AA class although it's in reality been over 60%. Piedmont is similar to Envoy except for them it's 3 or 4 per month that go to AA. PSA is completely different. They don't have a flow, but what's called SSP= Special Selection Process. That means they still need to interview for the job, and only 4 per month are selected to interview.