T-Mobile, AT&T, and Verizon all operate strong networks with 5G deployed across the Financial District, SoMa, Union Square, the Mission District, and the Embarcadero waterfront. The Fisherman's Wharf tourist area, Pier 39, and the cable car turntable at Powell Street are all well-covered.
The Golden Gate Bridge has strong outdoor signal at both the San Francisco and Marin County ends, and on the pedestrian walkway. Muni Metro lines have surface-level and tunnel sections.
BART stations have good platform coverage, with the transbay tube between Oakland and San Francisco having no signal. Haight-Ashbury, the Castro, Noe Valley, and Dolores Park all have solid 4G. Twin Peaks and Sutro Tower area have outdoor signal at the summit viewpoints.
The Sunset and Richmond districts toward the ocean have good 4G. Ocean Beach and the western edge of the city have reliable outdoor coastal signal.