Does an SP901-E actually amplify a signal or merely split the signal for use in multiple strips. I have a situation with 20 feet from ESP to LED strip. I was thinking ESP>>level shifter>>SP901-E>>>>>>LED strip. Is the SP901-E a waste in this situation?

Actually it’s a some kind of signal repeater. So inside it should be a logic which repeat and distribute whatever your controller will send to it. It’s up to you if in your application it will help.

If you only have 1 LED strip, then the SP901-E is not useful in your situation - especially with a level shifter on the ESP data pin.
If you want to drive multiple LED strips from a single WLED instance, using a single data pin, the SP901-E can help you out. You can use either 5V or 12V strips (but not different voltages together).
Inside, it probably has a level shifter with 4 inputs of the level shifter connected to the data pin of the input, then driving the various outputs.

Thanks for the clarifications. Repeater and distributor is perhaps better descriptors as opposed to amplifier.

No you can’t mix a power only same voltage different type of led strips. But I have a doubt it’s very useful because RGB order can differ from strip to strip.