Is it a bit or abit?

Excuse me, could you please clarify something for me? I've come across the phrases "is it a bit" and "is it abit" quite often, but I'm not sure if they're interchangeable or if there's a subtle difference in meaning. Could you explain the proper usage of each, and perhaps provide some examples to help me differentiate between them? I'd really appreciate it if you could take a moment to clear up this confusion for me.