How did the first manned flight re-entry happened?

Gagarin's Vostok 1 mission was entirely controlled by automatic systems or by ground control. In an emergency, Gagarin could have had manual control by entering a code, but he never did.

It appears the initiation of decent of Vostok 1 was done automatically,

07:25 UTC

The spacecraft's automatic systems bring it into the required attitude (orientation) for the reentry engine firing, and shortly afterwards, the engine firing occurs. This takes place over the west coast of Africa, near Angola, about 8000 km from the desired landing point. The liquid-fueled retrorockets fire for about 42 seconds.

Vostok lands successfully.

Prelaunch photos of Vostok 1