Let's put our thoughts in order ...
Ohm's law is about linear (constant) resistance. This means we must not allow warming. In this arrangement we begin to experiment.
First, we connect a voltage source to the resistor and begin varying the voltage across it. As a result, the current through the resistor will proportionally vary - Iout = Vin/R. If we vary the resistance, the current will inverse proportionally vary - Iout = V/Rin.
Then, we connect a current source to the resistor and begin varying the current through it. Now the voltage across the resistor will proportionally vary - Vout = Iin.R. If we vary the resistance, the' voltage will proportionally vary - Vout = I.Rin.
In all these experiments, the power will vary since Pout = Vin^2/R or Pout = Iin^2.R. If you want to keep it constant, change the resistance in the respective direction. This means there are two input variables in Ohm's law - Iout = Vin/Rin and Vout = Iin.Rin, or the resistance has become "dynamic".
Such tricks are used to make non-linear resistors that keep constant voltage (e.g., a Zener diode) or constant current (e.g., a transistor). It can be seen from the IV diode curve and transistor output characteristic.
Of course, we can keep constant power according to Pout = Vin.Iin. This means to connect a voltage source to current source. Thus the voltage source will set the voltage across sources and the current source will set the current through them. More precisely speaking, in this arrangement only one of the elements is a source; the other is a load implemented as a non-linear resistor.
Now, to keep constant power, when increasing the voltage, we decrease the current and v.v. But actually we can do it only by changing the resistance (there is no other way to change the current or voltage). That is why the arrangements above are more suitable for the purposes of intuitive understanding.