No — SOLAS does not apply to every ship.
SOLAS mainly applies to ships engaged on international voyages, with chapter-specific scope (often cargo ships of 500 GT and above, and passenger ships).
Many warships, ships not propelled by mechanical means, wooden ships of primitive build, pleasure yachts not engaged in trade, and fishing vessels may be outside certain provisions. Administrations may also grant exemptions where allowed (see Chapter I and relevant chapter exemption clauses). Always check the exact chapter application regulations and the ship’s certificates.