To install GNOME extensions, the more widely used way probably is to use official GNOME extensions index after some configurations (very detailed explained here).
Nevertheless, when it comes from install them via command-line the explanation indicates that just to extract the appropriated-version zip file under ~/.local/share/gnome-shell/extensions/ and restart the shell would be enough.
However, I got my doubts about it. I mean, when we turn on the switcher at extension's page in extensions.gnome.org, and click to install it, is just this that is done?
I have scripted this process to automatize the extensions installation after upgrade to a new system-installation of 20.04 Focal Fossa, and observed some extensions weren't available, even from site perspective, even after whole system (and consequently shell) restart.
So I came here to clarify, how to properly install a GNOME extension via CLI? Has something changed about this in GNOME 3.36.2?
Well, It has went right after sometime!
First, as mentioned by pomsky, some of the extensions were not available for Focal Fossa's GNOME 3.36.2 yet. It was the case of ShellTille and Hot-Corn-Dog, although the second one is also reaching an error when we try to install it via GNOME extensions index GUI alternative
And, answering heynemma, even after have restarted GNOME Shell I haven't found all extensions properly installed first, however they were now!
I couldn't track what was the real problem. I have run the extensions automated installation right after had finished 20.04 system installation, so maybe something disturb it. But now, after uninstall all them and tried again, It run accordingly.
Thanks for the help anyway! Is very useful to be able to count with the help-guided well-intentioned community of Stack Overflow