Basically as the title says. We have semi frequent power outages where I live. The noise machine in my daughter’s room goes out and wakes her up. If I were to buy a USB powered one, plug it into a power bank like one of those 10000ma ones you get for charging cell phones, would it have continuous power. Basically like a cheap UPS
Most power banks can either provide power, or be charged, not both.
They actually make what you want however, you can get usb UPS modules. They are intended for raspberry pi power, but just present a usb socket. They hot switch between external usb and internal batteries, as required.