When you purchase an OS map nowadays, there is a scratch-off panel on the inside front cover. Scratching off the panel reveals a printed code. You are meant to make sure that the panel is intact before purchasing the map because a scratched off panel may indicate that the code has been used by someone else. All codes are usable only once.
You then go to a specific Ordnance Survey website on your device be it a tablet, phone or PC and enter the code. You create an account and enter the code on your device. If the code is accepted, OS download a digital copy of your map to the device. Note that the download is the exact map that you purchased and you are not provided with an option to download any other OS map.
The whole thing is nothing to do with the Play Store. It's strictly a service offered by the Ordnance Survey. Yes, the digital download does take up quite a bit of space but there's no way around this apart from clearing space on your device. It's definitely possible to download to a PC or laptop but the idea is that that the digital version is portable - lugging a laptop up Snowdonia is a no-no!
It's not classed as a free app as you've paid for the paper map. The purchase just entitles you to use the app if you wish.
I have discovered one problem with the whole idea though which I'll complain to OS about when I get a minute. I bought a paper copy of the Explorer Snowdonia map for a trip there last week and duly downloaded the app to my phone. The following day, I was given an identical map with the scratch panel intact. Now if I scratch off the secon maps panel and enter the code, it will download an identical map to the one I already have on my phone. I think this is unfair and potentially useless under these circumstances. Someone purchased the second map after all and an alternative map should be selectable.