Majority of Commissions come through ROTC programs from colleges throughout the states. Most military academies are fairly elite, think Ivy Leagues, and graduating them essentially get you fast tracked in promotions (probably, I'm pretty far removed from this). Promotions in the military are weird and depend on different things, but the good 'ol boi club is still very real here, so networking and being alumni does have a large effect.

It's not only rich people that commission, I meant that rich people who do join the military, will commission. Still commissioning is largely done by folks from better off families because of the cyclical nature of these things - if your pa was enlisted, you'll want to go enlisted. But further than that - the ability to actually get a degree or an ROTC scholarship before joining the military means you aren't joining the military as a last resort, you're joining because it's what you want to actively do. And most people that get the GI Bill after don't want to join the military again after being enlisted.

There's always different cases, I know people that enlisted with degrees because they simply don't want to have the responsibility of commissioning.

As for people that enlist first, theres a HUGE sentiment against comissioning after enlisting (tongue in cheek called joining the darkside). It's not an openly shared sentiment, but it's something I'll attest to in many different areas. You get treated different if you let it be known you are wanting to try an commission, even if you aren't simply being an ass and saying you're gonna be x's boss.

Finally, it's significantly harder to commission than to enlist... and if you are trying to commission after enlisting, it is even more difficult. Out of 120 people that were signed up for Tx State OCS, 38 made it to Phase 1 (The actual start) after phase zero... 18 graduated... 17 commissioned.