There were clamps on the base, holding it down until ignition, and it was attached at a few points on the side to the launch tower, the red structure at its side.
At ignition, the base clamps released, and the side clamps swung back and sideways
The recent Apollo 11 documentary shows the side clamp release, as do other documentaries, whether official release or on YouTube.
There’s also a really good slow-motion video of the base at ignition, on YouTube.
Base clamps;
Side arms;
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