Multiple Input supply switching
If your intention is to run from battery until USB is plugged in, and the battery voltage is less than USB power (5V +/- 5%), then you might want to look at ideal diode controllers.

I have used this in the past (LTC is not the only company making this sort of thing, it is just what I happened to use in the past).

There is a nice overview of them at this link

If you want to always switch to USB power and the battery voltage may be higher, then you would need to sense the USB power and use a couple of FETs to control the power path. I would not expect you to have large losses in the FETs, incidentally. This approach may need you to insert some dead time between the switches; i.e. a break before make switch.

You could, of course, use a battery charge controller which usually permit the system to automatically drive the circuit from the charging source while recharging the battery (should you be using a rechargeable battery). You will need to take care to not exceed the USB current draw limits; charge current &lt;= USB(Imax) - I(circuit).

HTH