Simpler time, simpler jokes.

It’s called absurdity. It’s funny because it’s obviously not true, and also funny because of the undertones that it’s a clever way to get horny teenagers to study physics who really just want to see the photos.

It's a non-sequitur.

Everyone knows Alanis Morissette is a master electrician. That's why she has one hand in her pocket, while the other is probing a circuit.

Anyway, it's kind of Ironic [1] that the dude from Queen is an astrophysicist, and Britney Spears is writing Semiconductor Physics tutorials. Something something music and math.

[1] Sorry

Others have pointed it out, but it’s the juxtaposition of the fact that she’s definitely not an expert in this subject with a lesson in the subject.

There’s some subtle bits to the humor depending on how charitable you’re feeling. It might just be absurdist, as in “Blackbeard’s guide to astrobiology”, or it may be more mean spirited and playing on a belief that she is not intelligent.

TL;DR - the joke formula is just:

subject=…

person_not_familiar_with_subject=…

joke=“${person_not_familiar_with_subject}’s guide to {subject}“

And the amount of implied cruelty in the comparison is variable.

"I don't get it."

Same as “the narwhal bacons at midnight” or 67.

At the time she was ground zero for all of pop culture and pretty much every single man’s crush. When that website came out it was funny because we were all looking at pictures of Britney anyway so may as well learn some physics at the same time.

Btw, she really went downhill once she fell out of popularity. I think she has a life put back together finally but she got a raw deal. I’m rooting for her.