Many beginners seeing exchange as wallet it because that where they first bought Bitcoin., so they do have this belief. And again exchange making everything looks simple and easier, they don't really stress their self to think about who controls the private key.
Custodial or self custodial wallet, it's all easy and simple.
It's only different when people want to learn, see importance of private keys, full controls or they don't know about these important things and are ready to depend on centralized exchanges while no control for them.
That why I would advise that beginners should understand self custody first. Once they understand that them holding Bitcoin means them controlling their private keys, from there they know the differences that exchange it for buying and selling not that it keeps your Bitcoin. This will go long way to help beginners avoid some mistakes
When people understood very big and vital differences between custodial wallets and non custodial wallets, they will naturally use non custodial wallets to store their coins. Especially if they don't have any plan of trading with their coins, with the purpose of holding, non custodial wallets will be their only choice.
Write down your seed, why?This seed is the source of all your keys and addresses. Therefore, it allows you to recover all of your bitcoins if you end up losing your wallet for whatever reason.
But this also means that anyone can steal your bitcoins if they gain access to your seed, so it's vitally important that you keep it secure.