I understand how buck/boost converters change the output voltage, but how is an MPPT converter able to control the input voltage/current while keeping the same output voltage? I understand the theory of MPPT, but not the implementation, and every search for "how MPPT works" leads to how the theory works.
An MPPT basically works by attempting to present a solar panel with the ideal "resistance".
The basic idea is that for zero current (open-circuit), they provide maximum output voltage, and as you increase the current output (i.e. you go from infinite resistance to zero resistance) the voltage of the solar panel decreases. Somewhere in this range, there is an ideal amount of "resistance" that when connected to the solar panel, will extract the most power. Or, in other words, there is an ideal amount of current to "pull" from the solar panel that will provide the maximum amount of power
The way MPPTs do this is by having some kind of DC-DC converter, such as a buck-boost converter, in which they carefully control the output current of the solar panel (i.e. the input current of the MPPT) such that the maximum power is obtained. Essentially, they behave as if they were variable resistors, trying to find the best spot to consume the most amount of power. Of course, this inevitable makes the output voltage variable, as this input current adjustment will come from the DC-DC converter's duty cycle.