Teams Manager becomes a key factor in team creation once it's installed. Every team created through it is accessible in one place. Since older teams were created outside of Teams Manager, they're considered Unmanaged.
Unmanaged teams can be converted into Managed ones if you have the Enterprise plan.
Here's how you can do that:
Navigate to Settings > Expert mode.
Select Unmanaged teams.
If you'd like to exclude some of the teams from the conversion, you can do so by clicking the "+ Manage excluded teams" button.
Consider enabling the "Converting unmanaged teams creates a request" toggle to evaluate each converted team individually prior to approving it.
Once you're satisfied with the settings, click on the "Start converting teams" button, and the bot will begin the conversion process.
If you went to the Unmanaged Teams menu and cannot utilize the options mentioned above, you're likely not using the Enterprise plan. Consider contacting your account manager to get an upgrade.