Multiphase flowmeters and related methods are disclosed herein. An example multiphase flowmeter includes a microwave transmitter to transmit a signal through a fluid, a microwave receiver to determine at least one of attenuation or phase shift, a sensor to obtain at least one of pressure, temperature, or differential pressure, an intermediate-output generator to determine intermediate flow parameters, and a flow rate generator to determine a flow rate for respective phases in the fluid, and a model selector to select at least one of a physics model or a machine learning model for determining the intermediate flow parameters or the flow rates, the physics model to determine a first value of the intermediate flow parameters or the flow rates, and, in response to an error not being less than a threshold, the machine learning model to determine a second value of the intermediate flow parameters or the flow rates.