Most of these countries have a designated local exchange where they can store their BTCs. Unfortunately, however, exchanges are NOT secured when it comes to storing our BTCs given that they do not have full custody over their private keys.
I do suspect that people in these countries exchange their BTCs for their equivalent local currency for their disposal. While for others, I do think they purchase a hardware wallet for safely storekeeping their BTCs.
People can have bitcoins by mining or buying it or working for it and they are all have freedom in choosing how they store their bitcoins. It's freedom of selection on Bitcoin storage method while the recommended method is in non custodial and open source Bitcoin wallets.
It does not depend on whether they are rich or poor by having many open source and free Bitcoin wallet softwares to use. For example, Bitcoin core, Electrum wallets but there are more.
Storing bitcoins on any centralized exchange is dangerous by having only custodial wallets credited to your accounts. There is no Bitcoin private key you can have and you know "It's not your private key, it's not your bitcoin".
Hardware wallets.
Open Source Hardware Wallets.[GUIDE] How to buy a Hardware Wallet the right way.Recommended wallets in general.
Best Bitcoin wallets.Setting up a wallet.