Most visitors to San Felipe drive down from the United States via Mexicali, crossing at either the East or West Calexico border crossings and then heading south on Mexico Federal Highway 5 along the coast of the Sea of Cortez - the drive from the border to San Felipe is about 198 kilometres and takes roughly 2.5 hours on a road that is mostly fine and occasionally interesting in the way that Baja roads are interesting. The landscape through the Sonoran desert on that drive is genuinely striking and arrival into San Felipe from the north with the sea suddenly appearing is a moment that road-trippers who've done it tend to remember specifically.
From the US side the closest major city is San Diego, about 3 to 3.5 hours from the border, and the drive from San Diego through the Imperial Valley to Mexicali and then south is well-established enough that there's a whole category of Southern California residents for whom this drive is a practiced routine rather than an adventure. Los Angeles adds roughly another 2 hours. The Mexican insurance situation for US-registered vehicles is worth sorting before the crossing rather than at it - various online providers handle this quickly and it's not optional.
Flying in is possible via Mexicali International Airport which has connections from Tijuana and Mexico City and a few other domestic points, but the infrastructure for getting from Mexicali to San Felipe without a car is limited enough that most people who fly in rent a car at the airport regardless. Driving is genuinely the better option for this destination because the whole logic of being in Baja is tied to the freedom that having your own vehicle provides.
Getting around San Felipe itself once you're there is partly walkable from the hotel for the malecon and central town, and partly car-dependent for anything beyond that radius. Taxis exist and are cheap by any standard but the town is small enough and laid out simply enough that most guests figure out the geography quickly. For the desert, the dunes, the beaches south toward Puertecitos and the fishing spots that require some driving, a car is simply the correct tool and there's no realistic workaround.
Crossing back into the US from San Felipe requires driving back to Mexicali or using the Tecate or Tijuana crossings on the way north which some travellers prefer for routing purposes - wait times at Calexico can be significant on Sundays when the weekend crowd is returning and it's worth checking current crossing conditions before committing to a specific time on departure day.