How to network unlock

It depends on the carrier now >_<. Metro/T-Mo uses an app and does not use a code, AT&T PREPAID stole the T-Mo app method on newer phones so it's a crapshoot BUT usually pretty easy to tell because it has an app called "Device Unlock" but I assume anything new is the app just in case. Verizon is 60 days; for LTE phones, it's usually ~1 year but that's old phones. FCC licensing put Verizon in their place :-).

Sprint is dead and most of their phones are hard-locked except the GSM part which was not used here; it was an "international unlock" because they did it to keep their grip on the device. A lot (if not 90%+) of the "non-unlockable" GSM phones went out with the US 3G sunset so it doesn't matter since they're just eWaste now. For the survivors that predated the 2015 CTIA deal, the same issue applies. Anything post CTIA is fully unlockable but it's a pain because Sprint was hard-headed to the point we used to pay people to do the dirty part as it was faster. Though I THINK that T-Mo is letting people unlock them under their policy now since the Sprint merger I'm not quite sure but not sure if it has an active EIP leftover from Sprint. The problem with Sprint phones is their SW and radio alterations make them less than compatible with other networks, even today; unless your carrier uses T-Mobile towers (and it has the golden bands), it will either struggle to work well, you will lose features (or both with the case of LG devices). One source of trouble with LGxSprint phones is the VMM usually only works on Sprint and doesn't interchange like today when you unlock anyone else's device (as long as it isn't a proprietary app and the dialer VMM is blocked). If LG wasn't all too happy to bend over to the dollar here then their phones wouldn't be even harder of a sell from Sprint. A lot of carriers do not take Sprint phones because they're a trainwreck more often than not and waste support time due to their (sometimes very) custom nature. They need to be shoved where the sun doesn't shine! Others may take it but do not expect support when it has issues once they see the cursed nature of it.
Don't spend much time on a Sprint phone that's over 3 years old -- a lot of them also use weird 5G bands T-Mo did not keep and permanently neutered them so they will forever be "4G" phones... BARF! It was either killed off with the tower merging or the phone lost 5G with Sprint devices. Use it as trade-in material unless the phone has standard 5G bands.

On the postpaid side, T-Mo is an app like prepaid. AT&T postpaid (post app on PREPAID) is still (mostly) unknown to me because the phones are so expensive it will be 3-3.5 years before I can get them network unlocked, and they are beginning to lock them to the account signed the EIP so I can't reuse the phone if it has a balance. Verizon automatically unlocks it in 60 days now.

As far as the process goes per OEM? It depends. For example, Motorola has a lot of phones that use a "network unlock" procedure like T-Mobile but it can vary per original carrier too. For example, an ISP MVNO may just use a code while the big 3 may go either way. Most Motorola phones are so cheap most people buy them outright. $4-600 seems to be the magic number for phones primarily sold unlocked from the start :-).

Other manufacturers are even less consistent like Samsung. T-Mo Samsungs use the "Device Unlock" app or "Network unlock" method, AT&T can go between code vs "Network unlock" menu. I know the S20 uses codes but those are too old for me now. The S21-present can go both ways nor do I have a direct answer due to them having a balance in most cases. Google uses a profile like Apple to do this and puts a carrier-specific ROM on the device; much less awful, since it's a quick call or you have to use an app and it's just done since the carrier just requests that be dropped on Google's side. The problem with these current prices is I may never know with these new contracts running so long... 3-3.5 years to hide the cost!!! At this point, the only way I have a shot at it is if I play the waste bin lottery with family and get my hands on an S22/22+ (which I know is paid off so I can USE IT) or a phone which isn't EOL from the same lineup. Once I can do a few, I'll have a better idea. The problem is even they don't buy phones often now due to the cost; by the time I get to them, I'm rebuilding them with used parts due to cost vs. good examples and the updates stopped a long time ago. I can't squeeze blood out of a stone or make a moist napkin not crumple into shreds so how can I make a screen my dad broke yet again viable on a 2.5+ year old phone which is 40-50% of the way to a good one? Screens WILL TOTAL OUT SAMSUNG DEVICES, which both of them are loyal to. As of late, I've only been getting them when the screen is flickering, the glass is busted and the phone is no longer worth fixing (mostly). See here: How hard is it to restore the water resistance? :(. I will never find out personally because I do not finance phones; I'll put off the purchase until I can either pay all at once at the Apple store (or Best Buy for unlocked Android phones) or wait until I find the right deal on a USED one with good storage onboard. People are keeping their phones for longer now and when they hit the market they are worn out, old, or destroyed. This is even family I know throw them away like trading cards. As a result, I will not know with (then current) AT&T postpaid for sure for at least a few years. I am truly at the point I just do not care to find out as I am not waiting 3 years for someone to pay off a phone that's carrier-locked to unlock it and get the answer. I'll just buy the thing unlocked from the manufacturer and be done with it... I'm done for my sanity.

The cost of phones and length of EIPs make it such that carrier-locked phones are only for two people these days: People who are loyal to their carrier, or companies looking to lease phones to write them off on their taxes. Everyone else? Don't pass go and sign the EIP contract... Pay cash for the UNLOCKED variant since a lot of them are the same with carrier customizations and a SIM lock today; they use the support line to catch old timers who don't know they dump the problem on the device OEM today; service centers are dead.

The rest of us who buy used? We've responded by not buying your locked phone on sites like eBay if we want them unlocked, and killed your resale over this garbage. I've tried three AT&T PREPAID LTE phones (Moto, one delayed, the other was instant). LG was just as bad... 1 good, 1 delayed; that was it for me. I used to be fine with locked phones with minimal bloatware but 2 of 3 being a PITA made my mind up; to buy it that way.

This is how bad AT&T is now; I looked at varying models on price and likelihood but S20 is too old; S21+ (no seriously; all 3 IMEI#'s I wanted to buy and try to unlock with these newer $1k "debt generation" phones funded with bill credits or virtual Visa cards; all 3 had the SAME ISSUE):