Not wishing to teach Granny how to suck eggs but here's a little about NMEA.
Most modern GPS/ plotters support both NMEA 2000 & the older 0183. Whilst 2000 is quicker & designed to operate with more instrumentation the add ons tend to be expensive, for example you need a backbone wiring loom then connectors for everything you want to add
I prefer to use 0183, here's an example:
If you imagine your GPS/ Plotter as your core device, It is capable of talking to & listening to multiple others. Your DSC radio listens ie it receives position data from the GPS, it doesn't need to talk back. Autopilot also listens to the GPS telling it where to steer, it also doesn't need to talk back. whereas your Fish-finder/ depth gauge talks to the GPS, it doesn't need to listen.
With that said wiring becomes much easier.
If it's a listening device connect it to your GPS NMEA out or TX + & -
If it's a talking device connect it to your GPS NMEA in or RX+ & -
If it's a talking & listening device then use both.
If it talks incessantly, it's my ex wife !
You never know someone might find it this usefull