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How does boiling occur when vapour bubbles form inside a liquid?

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argon phase diagram — liquid Figure 1: Phase diagram of argon.

Boiling occurs when a liquid reaches a temperature where its internal vapour pressure equals the pressure exerted by the surrounding environment.1

Under these conditions, the heat supplied to the system provides enough energy to overcome the cohesive forces holding the liquid's molecules together. This allows for the formation of vapour bubbles within the body of the liquid, rather than just at the surface. Once formed, these bubbles rise to the surface and escape, a process known as vaporization.21

Because the energy added at the boiling point is consumed by the phase change itself, the temperature of the liquid remains constant until the transition is complete. The specific temperature at which this occurs depends on the applied external pressure; for instance, the boiling point decreases at higher altitudes where atmospheric pressure is lower.1