Curious why El Salvador uses Chivo app for BTC and what it means for future adoption in other countries

Are other BTC wallets like Muun allowed in El Salvador or do people have to use Chivo to use BTC? If only Chivo is allowed, is it a Lightning network? Can one Lightning network like Chivo communicate with another Lightning network like Muun? In other words, if I have BTC in Chivo or some other wallet, can I transfer that BTC to Muun with the same low fee and fast speed?

Outside of El Salvador, if I want to use my BTC to buy a hamburger at McDonalds, do I send my BTC to their Lightning network? What are fees like when sending BTC from a non-Lightning network to a Lightning network, and vice versa? For example, if I want to transfer my BTC from a Lightning network back to my non-Lightning wallet, what is the fee and transaction speed like?

If transactions can be made seamlessly from one Lightning network to another with low fee and fast speed, maybe BTC can work as a daily currency.