The Series 65 covers economics, investment vehicles, portfolio construction, and the legal and ethical duties of advisers under the investment-advisers-act-1940 and state law. No firm sponsorship is required to sit it, which is why people planning to launch an advisory practice often take it first.
Passing it does not make you an adviser. You still register as an investment-adviser-representative of a firm, or set up your own registered-investment-adviser with its own form-adv filing and compliance program.
Several states waive the exam for holders of certain professional designations. Waivers vary by state and change, so confirm with the specific state administrator rather than relying on a general summary.
Related: series-66, registered-investment-adviser, investment-adviser-representative, form-adv, fiduciary-duty