I'm pretty sure a lot of stock firmware is based on OpenWRT or used to be, though I'm pretty sure most of them lag well behind the current version. I haven't paid much attention for a while, but I think a lot were based on Kamikaze which is more than 10 years old now.
For the vendors with access to closed-source drivers and chipset info they can likely support devices not supported on the open source packages.
Edit: Per Wikipedia, "Qualcomm's QCA Software Development Kit (QSDK) which is being used as a development basis by many OEMs is an OpenWrt derivative"
It also notes Ubiquiti's wireless router firmware as being derived from OpenWRT, but I thought I remembered discussion of Ubiquiti being derived from a different open source distribution - unless perhaps the routers and wireless devices don't share a code base.
v1 to v2 upgrades the Flash (8MB to 16MB) and uses a slightly different AN+AC wifi chip. v2 and v3 seem pretty similar at a glance. v4 is rated at 12v 2a rather than 2.5a; using a completely different BGN(2.6ghz) chip and also different ethernet chip/switch. v5 is lower power still at 1.5a, but it's less obvious where that change happened due to lack of pictures. A guess based on the simpler antenna list is that it uses less antenna.
For the vendors with access to closed-source drivers and chipset info they can likely support devices not supported on the open source packages.
Edit: Per Wikipedia, "Qualcomm's QCA Software Development Kit (QSDK) which is being used as a development basis by many OEMs is an OpenWrt derivative"
It also notes Ubiquiti's wireless router firmware as being derived from OpenWRT, but I thought I remembered discussion of Ubiquiti being derived from a different open source distribution - unless perhaps the routers and wireless devices don't share a code base.