[binty]

Welcome to the fold.
You will find loads of help on here.
I would research aftermarket stereos with someone who has done it. The audio system works on a fibre optic loop, this connects the receiver (which is in the roof at the back) with the controls in the centre console and the amp under the drivers seat.
It is only really wired to the speakers from the amp.
The sound quality is very good with plenty of power as standard, but it lacks modern functionality for streaming, iPods, USB’s etc.
Quite a few people have used an optical interface which plugs into the loop then gives that additional functionality.
I have used the OASI one connected to a parrot mk3200 and some have used Grom, there may be others.
I have been very happy with mine, all the functionality is there, Bluetooth, streaming, usb, iPod, aux, and it feeds into the optical loop so is amplified by the Volvo amp not the cruddy parrot one. Retains steering wheel controls and mute for reversing sensors.
It is an easy fit.
Some other people have had good results with other systems too. Have a look using the search function on the forum.