"The GL.iNet OEM firmware is a fork of OpenWrt and thankfully is compatible with official OpenWrt sysupgrade images, so returning to OEM is done simply by flashing their sysupgrade image without keeping settings and vice versa."
With GL.inet the buyer can install their own OpenWRT images. The OEM OpenWRT fork is a means of installing the buyer's choice of OpenWRT image.
An OEM OpenWRT fork ("intermediate firmware") is (pre)installed, allowing a buyer overwrite it with an open source, GPL compliant OpenWRT image of their choice downloaded from openwrt.org or one compiled from source code downloaded from openwrt.org.
With GL.inet the buyer can install their own OpenWRT images. The OEM OpenWRT fork is a means of installing the buyer's choice of OpenWRT image.
For Cudy, another Chinese OEM, OpenWRT ToH refers to this as "Intermediate Firmware". See, e.g., https://openwrt.org/toh/cudy/tr1200
An OEM OpenWRT fork ("intermediate firmware") is (pre)installed, allowing a buyer overwrite it with an open source, GPL compliant OpenWRT image of their choice downloaded from openwrt.org or one compiled from source code downloaded from openwrt.org.
OpenWRT One is a Banana Pi board. Like GL.inet or Cudy, the Chinese OEM has their own system images.^1 Can a buyer reproduce them. 1. For example, https://docs.banana-pi.org/en/BPI-R4/BananaPi_BPI-R4#_system...