There are different types of PLB's, EPIRBs etc.
Some transmit GPS data from and inbuilt GPS, some need a GPS connected to them, and some are just a beacon.
They work by transmitting on one of the assigned frequencies, and there are Satelites and aircraft etc equipped to recieve the signals. If the signal has GPS data, it can be quicker to know where you are. If it doesn't have GPS data, then the Satelite will narrow the signal to an area, then search aircraft/vehicles will search in this area to narrow it further.
There is some more data from the ETS home page with tests with some of the 406Mhz models which I suggest you read.
I suggest you do some research into what other people are using in similar conditions or use. They will have already sorted out some of the reason why they purchased the model and brand for their use.