RC phase shift oscillator
The amplifier, together with the RC networks provide greater than unity gain at some frequency, that's the frequency it oscillates at.

How does it start? All circuits have broadband noise. The noise is amplified, and the oscillation builds exponentially.

Eventually the oscillation amplitude is large enough to hit some limit. Either the amplifier hits the rails, so loses gain, or in a much tamer process the amplitude is detected and used to turn the gain down to unity. Either way, the oscillation stabilises at some level.

These oscillators usually do not start when simulated, as a simulator has no noise. Typically you would use initial conditions to give it a kick.