No Stupid Questions

All the other answers are wrong, the real reason is that the browser does black magic by computing the number of pixels divided by zero.
Through reverse osmosis, this can be found to collude to the RGB pixels of the bottom left corner which if the same value can be determined that the color is black.
See the color code for black is #000000 which when turned into RGB is R:00 G:00 B:00. It then reversed the process to count all the white pixels. This can rule out nearly 2/3rds of images and the other ones are outsourced to an AI company.
The company in specific is the company that was just 500 people in India who all shuffle the images on a HUGE table. This is very slow, so the CEO speeds it up by giving them an IV tube filled with 50/50 redbull and radioactive sludge.
Once the AI finds a correct match the images are inserted into a vacuum tube to the basement where buddy the elf is having a dance party with the tough mail people. Due to this, there is only a small chance it will happen to float into the correct output vacuum tube. This is why TinEye is extremely unstable in giving accurate results.
Once the output is given it sends the letter to the local library to photocopy it by the sweet librarian named Edna. Once the image is found it is then e-mailed to HR to be double checked ever since those teenagers scanned their butts that one time.
It is then sent from Edna's computer to a USB stick when gets stuck in car traffic because of the train festival located in town once every decade.
Once it gets back to TinEye HQ the janitor plugs it into the computer and looks at it even though the IT guy told him not to. The IT guy sees this on the alert system he installed last time and takes it from the janitor to deal with it properly.
The IT guy gives it a ticket number #538,221 on a sticky note then manually uploads it to TinEye.com to get to the customer.
He then closes ticket #538,221.

JPEG works in 8x8 pixel blocks, and back in the day, most JPEG images weren't all that big. Each 8x8 pixel block (64 pixels per block) could easily and quickly be processed as if it were a single pixel.

So if you had a 1024x768 JPEG, then the fast scanning technique would only scan the 128x96 blocks, not necessary to process every single pixel.

Of course the results could never be perfectly accurate, but most images are unique enough that this would be more than sufficient for fast scanning.

https://vincmazet.github.io/bip/filtering/fourier.html

There are ways to encode images that make it easier to isolate differences in cropping and resolution and rotation. Like how if you wanted to search for color filtered images you could just throw out the color and compare them in black and white.

They had the AI models of those days.

Looking up similar images and searching for crops are computer vision topics, not large language model (basically text predictor) or image generation ai topics.

Image hashing has been around for quite a while now and there is crop resistant image hashing libraries readily available like this one: https://pypi.org/project/ImageHash/

It's basically looking for defining features in images and storing those in an efficient searchable way probably in a traditional database. As long as they are close enough or in the case of a crop, a partial match, it's a similar image.

What you're looking for is the history of "computer vision"