This is a primitive kidney of a microscopic animal, Asplanchna and you are watching it produce urine! This is a bit challenging to explain I hope I do a good job, please let me know after reading.
Asplanchna is a bit larger than half a millimeter, and looks like Casper the Friendly Ghost. It's microscopic but it has many organs like guts, an ovary, a bladder and this simple kidney known as the protonephridium.
The four finger-like structures, each with a little wiggling flame kinda thing inside, are called the terminal organs and the wiggly structures are actually called the flame cells because of the flickering flame-like appearance.
In the second clip, you can see that a big portion of Casper's body looks empty, that's called the body cavity and it's not empty at all! It's filled with body fluid and Casper has to maintain the composition of the body fluid to continue its life.
So those four finger-like terminal organs are inside the body cavity and reabsorbing the body fluid for filtration. I couldn't find concrete information on what causes fluid to flow from the body cavity into the terminal organs, but a couple of sources indicate that it may be the flickering of the flame cells that causes a negative pressure and sucks the fluid in for filtration.
And each of the terminal organs connects to a hollow tube, I said the terminal organs were like the fingers and you can imagine the hollow tube as coiling in the palm of the hand. In this tube, Casper actively chooses what to reabsorb or excrete; if it's put into distilled water where there are no dissolved matter, the water flows and fills Casper's body and the kidneys react to this by increasing the excretion of water and decreasing the excretion of sodium, potassium, etc.
And this part of the active selection of what to pee and what not to pee is energy consuming so there are a lot of mitochondria, the powerhouse of the cell, around this hollow tube providing energy for the process.
The hollow tube connects to the bladder, and the bladder contracts when it gets full and excrete the urine out of the body through a special opening! They pee like the kids in the public pools; quite often, every minute or two!
Thank you for reading!