What does it mean for bitcoin to be legal tender in El Salvador?
Does it mean all shops are forced to accept bitcoin or shops can accept bitcoin only if they want to? Logically, it doesn't make sense for it to be the first option because not all shops will have the infrastructure to (e.g. a small convenience store or grocery shop). If it is the second option, then I doubt many shops will opt for it.
Or does it mean products in El Salador are priced in satoshis?
Products definitely don't need to be priced in satoshis and I think that most if not all are still in terms of USD.
It's just that people are able to always get out of a transaction paying with BTC - the merchant can't technically refuse.
I'm sure that practically it's a bit different since not every single transaction ends up in court, but this does allow much greater adoption to happen.