It helps to know that Lerp and moveTowards are only a line or two of code each. They aren't doing anything special. The angle versions only have an extra line or two checking for wrap-around (ex: 350 degs is only 20 away from 10 degs):

A=Lerp(B, C, 0.1): A = B +(C-B)*0.1 // A is 10% of the way from B to C

A=moveTowards(A, B, amt): A=A+amt; if(A>B) A=B; // add amt to A, but not more than B

Using your current value as an input a=Lerp(a, b, 0.1); allows you to call it repeatedly, moving you 10% closer each time. That's a cute trick. It's pretty much the same thing as saying speed=speed*0.9; to create friction, which affects you more when you are going faster. Of course, instead of Lerp, you could just write: a=a+(b-a)*0.1;

The two ways of moving at a constant speed are 1) adding a constant value to your angle, using RotateTowards, 2) Fixing start and stop, and changing the percent from 0% to 100%, using Lerp. The adding method is easier, but can get math errors (0.1+0.1... ten times is ten tiny rounding errors away from one.) You'll get there, but maybe a frame more or less than you really should.

MoveTowards method:

  // do this anytime to start it:
  target = whatever

  // each frame:
  angle = Quaternion.RotateTowards(angle, target, speed*Time.fixedDeltaTime);

Lerp method:

// do this anytime to start it:
float pct=0;
Vector3 start=transform.rotation; // save start angle
Vector3 end = whatever

// each frame:
pct=pct + speed*Time.fixedDeltaTime;
if(pct>=1) pct=1;
else { transform.rotation = Quaternion.Slerp(start, end, pct); }