Once I have installed the "OpenWrt Snapshot Install URL Firmware", do I install the "OpenWrt snapshot Upgrade URL Firmware" and then the "OpenWrt Install URL Firmware".
No need to install anything else. The factory image is for first installation of Openwrt over the vendor firmware. The sysupgrade firmware is used for all the other upgrades from Openwrt to another Openwrt.
Careful however as the snapshot is not a stable version, does not have Luci by default (you'll have to login with SSH and install it), and once a new snapshot is released you won't be able to install packages due to broken dependencies. So if there is no specific reason, prefer a stable release.
Anyway, as you see there are more than one way to revert to stock without having to open the case, if this is your concern. Serial connection remains the last resort.
Thank you all for your valuable advice, because my knowledge of Linux is nonexistent, I am a Windows user. You make this forum a mine of information.
Bests regards
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