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The EGR valve and cooler used to be one item.
But following this kicking off the parts can be ordered individually.
The valve allows the gasses in, the cooler well cools them before they enter the inlet manifold.
The EGR cooler if you lift the bonnet, take the engine acoustic cover off and pull the noise insulation at the front of the engine is the big metal shiny "brick" thing at the top of the engine at the top. The EGR valve is at least in part that you can easily see the black plastic cylinder bit on the left of the cooler.
In essence the cooler fails allowing coolant to enter into the inlet manifold eventually mixing with carbon that will have come from the EGR valve gasses coming back round. Because there is no solvent in there these build up and combine with the coolant? you get a mess, combine that with heat though more so at DPF regeneration time then you can have a nicely melting inlet manifold made of plastic and as BMW have previously put it a "thermal event".
Regarding the age of cars it seems never ending. But there is a lot of cars to do parts are scarce in some cases as well not helping things.
Either way the EGR valves/coolers on the N57 (and B47 for that matter) are known weak points. In honesty they all are across any vehicle.
The issue is that this issue is not particularly obvious until it properly becomes an issue (unless you are watching coolant use etc.) or smoke coming from the exhaust due to coolant entering the engine and exhaust system.
Honestly of all those symptoms mentioned -
Low coolant warning - the first proper sign you have the issue!
check engine / drivetrain warnings - rarely heard this one and this is part of the issue there is little obvious signs before the issue makes it self known
Loss of power
Exhaust smell while driving, whistling from engine
smoke from engine compartment
They forgot the smoke from the exhaust...
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