Acts done in an official manner are immune. So for example if the president orders assassination of political leader of another country (what Trump did with Iran’s Suleinami (I’m probably butchering name)). Protects president from prosecution for murder or whatever if there is evidence it was done in the interest of the state.
Another example is something Biden / Trump and even Hilary are guilty of. The misuse or mishandling of classified materials. Since they are acting in an official manner, it isn’t a crime like it would be if a normal citizen mishandled the documents.
Acts done in an “unofficial manner” are not immune. So let’s say a Mr President does some insider trading while president to enrich himself personally. That presumably would still be illegal and he could be charged.
So who decides what is official and what isn’t? The courts. Lower courts make a determination and presumably it would go up to the SC if necessary.
It’s an interesting question. For example- Reagan’s Iran-Contra episode. Where his administration was smuggling cocaine in order to get money to covertly supply weapons to Iran. Would that be official or unofficial?
I think people need to realize the president has had broad powers to do a lot of dubious things for decades. This doesn’t necessarily increase or decrease his power, but creates a potential pathway to either prosecute or acquit him. Whereas before, it always stayed in the legal gray zone (in Reagan’s Iran Contra)