When you install an add-in for your web browser, it generally asks for permissions to access your account. For example, if you have an add-in that can filter your email and check for important appointments, the app will first ask you to click a link in order to grant it access to your calendar.
Even if you subsequently remove that app or add-in or extension, those permissions remain. So over time, the number of companies which has access to your account will grow.
I always make a point, at the end of each year, to review these access arrangements and delete those that are no longer required. And I recommend that you do too. In the case of a Google or Gmail account, just go to https://myaccount.google.com/security
and click on "Apps with account access". You'll then see all the apps and organisations which have access, and you can delete those that no longer need it.
In my case, I found (and deleted) 18 separate companies that could still access my Gmail account if they wanted to.