Is it a new build and is that just bare metal? Mine is the same but there was a similar discussion in our estate because not all the houses are like this. To me it looks unfinished, even if its meant to be like that.
The ‘white plinth’ as you call it just looks like a full width profile to accommodate the trickle vents rather than separate vents stuck on which is more common. Google ‘concealed trickle vent cover’ for an idea.
Its not missing anything, the window has been set too far in and thats why you can see the lintel above. A length of upvc trim will hide it but honestly whoever fitted the window should have finished it off.
