I have a couple of questions regarding wallets which will, no doubt, confirm my ignorance...
Once I've chosen a wallet, what happens? Do I have to put money in it to be able to purchase coins? Do I have to 'activate' a wallet by paying money in?
If you use a real bitcoin wallet- often we refer to "real" wallets as wallets where you have control over your private keys, and the answer is NO. you download it for free (it's ALL open sourced software), and then you either buy Bitcoin on an exchange, and transfer it to your wallet, or work for it some other way.
"Real" bitcoin wallets are free, open source, and never deal with real money (dollars). On some web wallets there is an integration with the fiat payment system, (Blockchain.com). But with the context of your story, i don't think this is related.
Do I have to put money in to cover transaction fees? Do I have to link the wallet to a specific exchange or is that done depending on which wallet is used?
Bitcoin itself is used to cover transaction fees. I have yet to hear from a wallet that needs to be linked to a specific exchange. (Perhaps this is the case with Binance's BEP protocol, but that's completely unrelated i'm pretty sure.)
I'm trying to establish, in my own mind, exactly what the process is. Is there a step by step idiots guide to this? I want to try to understand so that I can identify the BS that is being fed to my brother.
You download software from Electrum / Bitcoin core, or compile it yourself.
Then, launch the wallet, generate an adress, and then you send bitcoin to it.
(The bitcoin is usually bought from an exchange such as Coinbase.com, or perhaps a P2P transaction with a site such as LocalBitcoins.)
Once the transaction is confirmed, no one but you has control over where and how it goes to its next destination.
So, in general terms, he is being told that his coins are available in a wallet (possibly not his? I'm not sure) but he needs to put in cash (using the term 'Overturn') to activate the wallet and release his funds. Apparently, he has 4.8 bitcoins in there but needs £6k to get them back. This appears to be complete bullsh*t to me but as I've said I have no experience of this.
I give it a 0.001% chance it's actually legit. No reputable business or wallet conducts business like that.