The first type I bought here for $2.31 apiece, including shipping.

The second one from the same seller was slightly cheaper.

Description of both mentions that they are used, untested, and the seller does not know how to control them. There is no mention of brand or datasheet. 

Even though these are no longer available, you can still find similar used servos, like this one.

While the larger servos have no branding, the smaller ones have UBTECH name on two sides. This name eventually leads to their offering of small servos. And here we go.

There is a table with some basic parameters, a dimensional drawing, and a datasheet for each servo. I could not open the datasheet, so I'm stuck with whatever info I can get without it.

To illustrate some parameters, here's a rough comparison with SG90. There is a lot of variety when it comes to SG90 so take these figures with a grain of salt. 

Aliexpress description offers a rough outlook of the insides, but I took them apart to understand better what is behind the specs.

The larger 3kg servo has a magnetic encoder and Cortex-M0+ microcontroller. The board with motor is available without the shell for about a dollar on Aliexpress too(here). So when I eventually burn something, replacement is very cheap.

The smaller one uses a potentiometer for feedback. Here I couldn't identify any ICs. Here the board with the motor is also available for cheap.

Now that I have some idea of what I'm working with, I can start searching for a way to control them.