SOMETIMES the broadcaster will broadcast their main channel at 720 to squeeze another 480 sub channel, fortunately that is not often as I can't stand watching the sub channels and do not want to see a main channel degraded to 720.
I believe the reasoning of 720p vs 1080i on OTA stations is that 720p was better for sports action at that time. Interlace (1080i) updates the odd lines, then even lines, while progressive paints all lines at the same time. My ABC and Fox stations broadcast their main station as 720p, while CBS is 1080i. Local ABC only has one sub-channel.
For now, most ATSC 3.0 broadcasts are in 1080p because many of the local stations have agreed to have one tower as the "lighthouse" for ATSC 3.0 main channel broadcasts; and that one tower/station that has multiple sub-channels have them moved to the other stations as sub-channels, though you don't notice this because the sub-channel is identifying as the sub-channel of the main original channel. The one lighthouse ATSC 3.0 transmitter has a bandwidth limit and it may be sending out 5 to 6 1080p broadcasts. Eventually when the other stations get their own ATSC 3.0 equipment and move their main channel back to their tower/transmitter, it will open up the ability to send higher-res information....or they may keep it as is and rent the extra bandwidth for private use or have subscription service OTA. They are still experimenting with ATSC 3.0 and don't see it becoming main stream at time soon.
I dropped DISH back in mid 2000's and started using FTA satellite, mostly the Ku band. Also had a HD TV card and used MythTv to watch and record OTA, couldn't believe the picture quality and sound...felt like I was in the bleachers at the ball game (beer, beer here). As they added sub-channels, it seems that the sound quality went down and didn't get that feeling I was at the game.
But when the local internet company upped the speed from 10M to 50M, I started streaming stuff, like MLB. Then joined Netflix for a month or two, binged watched, dropped them, joined Prime for a few months and binged watched and drop them. Still do this every now and again.
Note that there is a lot of free stuff for streaming, like PlutoTv, Redbox, Tubi, etc. I'm using Plex and HDhomeRun for OTA recording. Also had Emby, but have yet to try JellyFin.