They're breakers. Like most breakers they pop to off and you switch them on again when they tripped.
Panel 424 Switch 18, 19 and 20 are normally "reserved" (meaning unused from factory), but that doesn't mean they haven't been used since (like your 19 and 20 seem to be). Follow the wire to be sure.
The Seatalk converter blue connectors are for the backbone. If there's something plugged into them that's fine, but the backbone must be terminated on both ends (one end is the other blue connector which has a terminator visible in it). No more, no less. The manual explains all of this quite clearly.
It's better to give the AIS a separate power switch. There can certainly be situations where you want to switch it separate from the plotter, e.g. when either one is misbehaving or you need to fiddle with the wires whilst underway (certainly has occurred here).
The extra switch on the panel looks like a custom addition. You get to follow those wires too. Used boats are full of treasure hunts like that!
Thingy 2 is the connector from the panel 424 switched positives to the boat circuits (i.e. cabin lights, power for radio, etc.)
Thingy 1 seems to be more of the same - we don't have that one, but you will probably find the numbers match with the numbers on the switch panel breakers. Probably slightly thicker wires, thus the separate connector block.
Thingy 4 I can't quite make out but it looks a lot like a blade fuse holder, in which case it may be for the windlass or a forced air diesel heater. Could be a relay too, hard to see. Have a better look at it and you'll soon know.
Thingy 3 is a RJ45 connector, most likely for Ethernet, which means most likely the Radar, unless perhaps there's a Wifi booster or some such around.