In the experiment of studying the relationship between current, voltage and resistance, what is used to control the current in the circuit

It depends on what equipment you use to do the experiment
The simplest way is to use sliding rheostat. When the voltage is constant, the sliding rheostat is used to control the resistance of the whole circuit, so as to control the current

The resistance of a conductor is directly proportional to the voltage and inversely proportional to the current, right?

No. the derivation of Ohm's law I = u / R is transformed into R = u / I. the meaning of this formula is that in a circuit, the resistance of a conductor is equal to the ratio of the voltage at both ends of the conductor to the current passing through the conductor. It is not the same as Ohm's law itself. Because the resistance of a conductor is the property of the conductor itself and cannot be changed with the current and voltage

Experimental design: when the current is constant, the resistance of the conductor is proportional to the voltage at both ends of the conductor

First of all, the voltage at both ends of the conductor is proportional to the resistance of the conductor
1. Design a series circuit, as long as there is a power supply, resistance (more than one), voltmeter (and on the resistance). Close the switch, observe and record the number of voltmeters, and then change another resistance to observe the number of voltmeters. We also know that in the series circuit, the greater the resistance, the more the voltage, so we can compare the number of voltmeters
2. Ohm's law I = u / R → u = IR, so the voltage at both ends of the conductor is proportional to the resistance of the conductor

When the current is constant, is the resistance in the conductor proportional to the voltage at both ends of the conductor

In general, the resistance has nothing to do with the voltage, which is only determined by the cross-sectional area of the conductor itself. Therefore, it can only be said that the voltage in the conductor is proportional to the current, that is, the resistance value. There is also a variable resistance related to the voltage

In the experiment of studying the relationship between current, voltage and resistance, what is used to control the current in the circuit