Is that something that can be done using the configuration files or the commandline?
For example: I want to bridge my 2.4ghz and 5ghz networks to one device (or group) to create a FW rule using that object.
This can all be done from the GUI. In version 21, network bridges (layer 2) are distinct entities from network interfaces (layer 3). In previous versions, an interface had a bridge option which would create an underlying bridge from one config block. This is deprecated.
Create a bridge device and give it a name (typically of the form br-xxxxx). If it is only going to contain wifi devices, don't choose any ports from the list.
Then create a network interface and use the bridge device as its physical device. Give the interface a similar name like xxxxx without the br. Configure an IP address etc. Add it to a firewall zone or go to firewall and create a new zone.
Finally go to the wifi config and choose the network name as the network for your wifi AP(s).