I was able write a test plugin and install it to QGIS (ver 3.24, Ubuntu 20.04 LTS). The plugin shows up in the dialogbox from the Plugins -> Manage and Install plugins...
menu. It is shown under the Installed
category but not activated (i.e. not checked) by default. So, it's not visible on the toolbar by default.
Is there a way to activate a plugin by default (so that it shows up in the main UI)?
Related question:
Installing QGIS plugin from command line or Python
There is a deleted answer/non-answer asking this:
How do you exactly activate the plugin?