The thermal energy transfer process starts with radiation from the flame, continues with conduction from the kettle to the water, and ends with convection currents within the water.
The heat energy transfer process described in the question can be filled in as follows:
The thermal energy transfer process begins with radiation from the flame,
then conduction from the kettle to the water,
and then convection occurs within the water.
The explanation for this sequence is that radiation is the emission or absorption of energy in the form of waves or particles through space or a material medium. This is how heat from the flame reaches the kettle. Once the kettle itself gets hot, conduction, which is the transfer of heat between objects in direct physical contact, occurs between the kettle and the water inside it.
Lastly, as the water in the kettle heats up from the bottom, convection currents are generated, which is the transfer of heat by the movement of the heated water itself, causing a uniform heating of the water.