Do I need MC authority or just a USDOT number?
Both. Every for-hire interstate motor carrier needs a USDOT number AND MC operating authority. The USDOT number is the FMCSA carrier identifier; MC authority is the FMCSA permission to haul interstate. Pure intrastate carriers need only a USDOT number (or sometimes just a state DOT number, depending on the state).
USDOT numbers are issued under 49 CFR §390.19 and apply to any motor carrier operating in interstate commerce, regardless of for-hire status. Private motor carriers (a company moving its own goods) need a USDOT number but do not need MC authority. For-hire carriers need both.
MC numbers are issued under 49 USC §13902 (motor carriers), §13903 (freight forwarders), and §13904 (brokers). The MC number signifies that the carrier or broker has filed Form OP-1 and is authorized by FMCSA to provide for-hire service. Without an MC, a for-hire carrier cannot legally haul interstate.
Some intrastate carriers operate under just a USDOT number with state-issued authority (CA intrastate motor-carrier permit, NY intrastate authority, etc.). The state authority replaces the federal MC at the intrastate level.
A common confusion: a private fleet (e.g. a manufacturer hauling its own product) needs only a USDOT, not an MC. The moment that same fleet starts taking outside loads for-hire, it needs both. The trigger is the for-hire/private distinction, not the state-line crossing alone.