Currently in many countries, the government has allowed bitcoin as an investment and I think its citizens are starting to feel safe and comfortable. In the past, when bitcoin was first launched, it was considered illegal in many countries, but now the government has been able to accept it, so I think in the future there will be many other countries that will legalize bitcoin.
Usually, BTC is considered an asset rather than a common investment for everyone. BTC and crypto are in a gray area: they are not banned but are also not officially regulated. Traders like this because they don't need to pay taxes on crypto income.
Currently, only El Salvador widely accepts BTC because they don't have their own fiat, they use USD so it's easy to accept BTC for the country with a specific and effective strategy. The same is quite difficult to apply in many other countries because BTC can have a negative impact on fiat. I can only hope that they will have an official legal framework to legalize the crypto market!
Just a difference in naming because they, have the same in my opinion about it. But indeed, the regulations on crypto, especially bitcoin, are very unclear because some prohibit, some don't, But still limit and some provide restrictions on their local rules.
Until now, there is no proper research on how many countries accept it widely and freely. Of course, as you said, it is elsavador, which we already know.
In fact, each country can regulate Bitcoin itself so that it does not impact fiat, which is actually very profitable with exchanges through local exchanges and can collect taxes and not interfere with fiat in general.