Filing a DBA, state by state.

A DBA, doing business as, is a registration of the name you trade under when it differs from your legal name or your entity's registered name. A sole proprietor uses one to trade as something other than their own name. A company uses one to run a second brand from a single entity. It registers a name on a public record. It does not create anything.

It depends on whether the name you are trading under matches the name on your registration, and on what the state you trade in requires when it does not. That is a state-level answer, which is why each page here sets out when a DBA is expected in that state. If you trade under your own legal name, or your entity's exact registered name, the question usually does not arise.

Either with the state office that keeps the business register, or with the county, depending on the state. This is the single biggest difference between one state page and the next, and it is the thing worth checking first. Filing at the wrong level does not produce a partly valid registration. It produces no registration, and the name stays unclaimed.

No, and they are not alternatives so much as answers to different questions. A DBA registers a trading name. An LLC creates a legal entity that stands between the business and the people running it. Registering a DBA changes what appears on your invoices. Forming an entity changes what happens when a claim arrives. Many businesses hold both, deliberately.

In most places, yes. States set a term for the registration and expect a renewal to keep it current. Terms differ, and your state page gives yours. Nothing dramatic happens on the day it ends; the registration simply stops being live, which is why lapsed DBAs tend to be discovered by a bank or a counterparty rather than by the business itself.

That is not what the filing is for. A DBA records that you trade under a name in a particular register. It is not a trademark, and it does not give you a claim over the name elsewhere or in other industries. If protecting the brand is the actual goal rather than complying with a naming rule, the question you are asking is a trademark question.