Not sure I understand that. Verizon Wireless, AT&T Mobility, T-Mobile and Sprint all have CONUS presence and more, they all have their own equipment and they are not MVNOs, although each has a prepaid MVNO arm. All four are 'primary carriers' in my opinion, it's just the first two have better coverage then the second two. Verizon and AT&T were able to grow their wireless brands by buying out smaller carriers, it also does help both are tier 1 internet providers with large fiber networks. Verizon and Verizon Wireless used to be Bell Atlantic and Bell Atlantic Mobile around here. Prior to that the landline operation was NYNEX, which was a RBOC from 1984. In 1997 NYNEX and Bell Atlantic merged, in 2000 Bell Atlantic and GTE merged to form Verizon and Bell Atlantic teamed up with Vodafone to form Cellco aka Verizon Wireless. It was only in 2014 that Verizon bought Vodafone's share of VZW. So for a brief period of time AT&T was the only 100% American owned carrier.
Here's a pretty nice write up on the consolidation of the teclos and cellular providers. Both Verizon and AT&T are descendants of the old Bell System, T-Mobile and Sprint are not.
T-Mobile entered the US market in 2001 by purchasing Voicestream which was in existence since the early/mid 90s.
Sprint, while it's existence dates back in the very early 1900s with the
Southern
Pacific
Railroad [
Internal
Network
Telecommunications] got into cellular in the early 90s by forming a partnership with regional provider Centel
Verizon Wireless' roots are with a Baby Bell and a partnership with the British
AT&T Wireless was bought out by Cingular, which was bought by SBC (a Baby Bell) and re-branded themselves as AT&T
A Brief History of the Rise and Fall of Telephone Competition in the US, 1982-2011
As for spectrum, Sprint has some pretty low spectrum, so they are trying. They have 800 MHz, not as good as T-Mobile's addition of 600 and 700 MHz, but 800 MHz should provide a nice coverage boost for areas that were previously 1900 MHz only.
For a full summary of LTE bands by carrier this is a nice summary
Cheat sheet: which 4G LTE bands do AT&T, Verizon, T-Mobile and Sprint use in the USA?