To install a custom extension the user has to enter the "installed" tab of the extensions page

It's semantically very strange. You want to install, so it's not installed yet, so it doesn't make sense to enter the Installed tab to do that.

I was talking to an integrator and their comment was "I would probably never find out that this button was there".

Start at the Extensions page and compare the Installed tab with the custom-extension installation flow. Identify where users would naturally look for installation, then verify that the revised navigation makes custom-extension installation discoverable without entering Installed.

Written by the indexing model from the issue text.