They can't do that unless they take your devices, gain admin access and install stuff onto it. You don't just get spyware installed your phone simply by entering a country.

Also the whole "if you aren't intending to do anything illegal" bit really reads like all the piece of shit bootlicking conservatives after George Floyd.

Except that is a whole different context. The argument doesn't work if you're a citizen of a country and granting your government more and more powers. It would apply maybe if you were a Chinese citizen. OP isn't talking about moving to China or installing a similar government in their home country. They are going on holiday. You can behave yourself and cooperate with their requirements for a few weeks. If you are really against a country having powers to check your phone and devices and such as a matter of principle, not because you've got anything to hide, then don't go.

George Floyd was an American citizen murdered in his own country by the powers that were supposed to protect him. Big difference.

Although I did take precautions myself, such as deleting my memes/downloads folder just in case I saved anything that could be offensive. But it didn't matter because they didn't check my phone anyway for simply being there.

China itself cares the most about public disorder and foreign influence. As long as you aren't intending on causing foreign interference in how they do things and are just going for purposes of tourism/adventure/meeting people, then you'll be absolutely fine. They don't really care enough about you to give you special treatment unless you are seen as a threat like that.

Doesn't the USA do the exact same thing?

You don't. I went to China a few weeks ago and my phone was never of any suspicion or brought into question.