Is it true that the USA is basically immune to foreign invasion?

Immune? No. But invading the United States would be enormously difficult and would nearly require the cooperation of Mexico or Canada (or both). And like most countries with nuclear weapons, we’d nuke the shit out of our own territory before we’d allow the country to be fully taken. And here I am not just speculating — that exact scenario in war games is often the only time that nukes are used.

Any foe that attempted to invade the US would pay such a heavy price it’d almost never be worth it and the chance of success is low. Also, the US is gigantic and its geography extremely challenging.

And then if you somehow as an invader won against the organized military, you’d have to deal with a well-resourced and capable resistance after that.

Good luck with all that.