LAGG setup and "down" detection

[bluerains]

Does anyone know how does LAGGs interface "detect" (or deem) the interface is "down"?  Does it simply check for a physical "interface" down to deem to be "down"?  (e.g. if the switch that is hook up to it lost power or some one cut the cable)

Or does it do some sort of "protocol check" or some sort of software check (e.g. able to ping 8.8.8.8 or some ip) and if no reply, then it deemed to be down and then switch over to the over interface?

The main thing is that I'm about to setup LAGGs interface on 2 HA-paired pfsense to 2 different "switch"  This way, if one of the switch goes down, it will not drag the pfsense down with it (because it seems CARP redundancy is only for if 1 of the pfsense actually physically goes down).

And the reason I ask "how" LAGGs deem a interface is "down" is because if the switch "power" (physically) does NOT go down, but simply "hung"  (e.g. software mess-up or), will LAGGs detect that and switch over even thought the interface is still "up"?

Thank you!

[heper]

Well static lags generally only detect link up/down AFAIK.

Then you have more modern stuff like LACP. its a bit more intelligent:
https://www.thomas-krenn.com/en/wiki/Link_Aggregation_and_LACP_basics

When multiple switches are involved you probably want STP (or brandspecific alternative)
https://en.m.wikipedia.org/wiki/Spanning_Tree_Protocol