Is Add-ons Menu Aggregator free?

Yes. Add-ons Menu Aggregator is released under GPL / FlightGear add-on and costs nothing. Neither it nor FlightGear has any paid content.

What do I need to run Add-ons Menu Aggregator?

FlightGear, which is itself free. Add-ons Menu Aggregator is an add-on and will not run standalone.

The PlayBound Launcher installs it in one click from FlightGear's mods list. You can also install it by hand — full steps are on this page.

Roughly 3 MB, on top of the base FlightGear installation.

Yes. The source is published at github.com/PlayeRom/flightgear-addon-menu-aggregator under GPL / FlightGear add-on.

Add-ons Menu Aggregator — Free FlightGear Mod · PlayBound