Footer can be managed by 2 ways in Airtech theme.
By default, the footer is powered using Elementor plugin.
So you can just go to Appearance/Header Footer builder to edit the footer using powerful Elementor plugin.
You can watch this video for visual info https://share.getcloudapp.com/L1uBRBYw
In case you do not want to use Elementor, you can enable the default footer, by going to Appearance/Customise/Airtech footer settings and disable the Elementor powered button. After that you can manage it normally by using widgets, by going to Appearance/widgets.