Over on his YouTube channel [Aaron Danner] explains biasing transistors with current sources in the 29th video of his Transistors Series. In this video, he shows how to replace a bias resistor (and consequently an additional capacitor) with a current source for both common-emitter and common-collector amplifiers.
A current source provides electrical energy with a constant current. The implication is that if the resistance of the load changes the current source will vary the voltage to compensate. In reality, this is exactly what you want. The usual resistor biasing arrangement just simulates this over a narrow voltage range, which is generally good enough, but not as good as a true current source.
As [Aaron] explains there are various advantages to biasing transistors with current sources instead of resistors, chief among them is that it allows you to get rid of a capacitor (capacitors are expensive to make in integrated circuits and often among the lowest-quality components in a design). You can also avoid losing some of your gain through the bias resistor.
The current source that [Aaron] uses in this video is known as a current mirror.
Current sources (or any other form of active bias) also can save a ton of power vs a manufacturer recommended circuit. RF amps frequently have recommended supply voltages way higher than needed because of the device voltage variation (since the bias current from a resistor depends on the device voltage). As opposed to a current mirror, you can also use an opamp to regulate the current into the bias pin and then you only need slight headroom over the device voltage.
The statement in the video at 8:44, that there would be no AC voltage at the emitter if the emitter resistor were replaced by a current source, is wrong.
Otherwise the video looks OK for a beginner’s education, but nothing more than that.
True, there could be voltage, although not current. Also, the common emitter circuit where the current source is placed in the collector, is not well explained. It would need some kind of feedback of matching Ic to Ie.