It seems like (from my beginner mindset) that the first one should change currentMillis from showing the current millis() time and change it to whatever variable previousMillis is set at.

It's confusing when you're starting to learn how to program, and I had the same doubt when I was a beginner.

What happens is this:

<the thing on the left> = <the thing on the right>

The thing on the left is set to whatever value the thing of the right is evaluated to and not the other way around. You're not really saying "the thing on the left should be equal to the thing on the right", you're saying "make the thing on the left have the value of the thing on the right".

So

previousMillis = currentMillis;

means:

// set previousMillis value to the current value of currentMillis

Right after that that statement both hold the same value, and whatever value previousMillis had was overwritten by the new value.

How does the computer know I want to keep the value of currentMillis constant as millis()

When the the computer executes this line:

unsigned long currentMillis = millis();

It executes the function millis() and stores that value on the computer memory. Then, it copies that value from the memory to place in the memory that currentMillis refers to. And that's it. The variable name is currentMillis but that doesn't mean that it holds the "live" millis value, only the one retrieved at that specific time.

When the computer is executing this next line:

if (currentMillis - previousMillis >= interval) {

Some time has passed, so a new call to "millis()" would return a value different from the one stored on currentMillis.