The military budget of the rest of NATO combined is much smaller than that of the US. ... Why would aliens be interested?

They still build some stuff and if aliens have come all this way why not check out the whole planet?

No no. The aliens are only interested in the largest military budget obviously. Because the US can definitely stand against a FTL capable alien civ

My guess is if there was aliens they would be so totally disinterested in us they wouldn’t even distinguish between military and civilian. Just zipping around taking their measurements of the mineral deposits.

You have nothing to back that up and your gut goes against the closest thing we have to evidence...

If I was an alien race sending probes far and wide I would absolutely focus my attention on hotspots of military presence rather than the relatively benign nature and civilian existence.

What does an alien military look like?

Minerals are plentiful. If anything is interesting about Earth at all, it's its lifeforms and cultures.