How does a USB bitcoin miner work?

I'm curious about how a USB Bitcoin miner functions in the context of cryptocurrency mining. Could you explain in a concise manner how this device operates? Specifically, I'd like to know how it connects to the Bitcoin network, how it processes transactions and mines new blocks, and if there are any particular considerations I should make when purchasing one, such as its hashing power and efficiency. Additionally, I'm interested in understanding the role of USB miners in the overall mining ecosystem and whether they're still viable given the increasing difficulty of mining Bitcoin.