It is essential for the longest life of any machinery, including our cars, to keep all fluids in very good condition. Engine oil, transmission fluid, brake fluid, gear oil, (where used, power steering fluid), coolant.
The video is pretty good. I give him a B. A much better way to test the transmission fluid, if you can extract 4 ounces, is a lab analysis.
https://www.blackstone-labs.com/engine-types/transmission/
His description of the types of fluid way missed the mark. There is little to no relationship between the specs for the type of fluid and the base of the oil. Conventional or synthetic bears no relationship to the type of fluid: MerconV, Dexron VI, Mazda's requirement for our cars. Then, there are the "universal" fluids that come close but may exactly meet no spec, or maybe do meet some specs and come close to others. Not to mention the transmission fluid supplements said to convert one fluid to meet the spec of another fluid--LubeGard is an example. Some modern fluids have such a high performance spec that only a synthetic or syn blend base oil can meet the spec, but there's much more to it than that. So...either use a fluid labeled to exactly meet or exceed the spec, or use a fluid from a trusted maker that "is suitable for" the spec knowing that it doesn't exactly qualify.
Some shops use an exchange machine where one chamber contains the new fluid and the other chamber is empty. The machine is connected to the transmission cooling hoses. Running the engine causes the transmission to pump the fluid through the hose filling the empty chamber where a diaphragm pushes the new fluid through the return hose and into the transmission. The concern is whether the new fluid chamber is clean of a non-spec fluid (a bit won't hurt), or even if they use the spec fluid at all, not just a standard fluid plus a supplement. A shop without the machine may just disconnect one hose and put a bucket under it. Run the engine and pump a couple of quarts of fluid, clamp the hose with vicegrips, add a couple of quarts into the fill hole, pump more, add more, until a case of fluid has been used. I wouldn't do it this way. Starting and stopping the engine is a pain, but it works well. No, you can't suck the new fluid in...there is no internal pump pulling the fluid back to the transmission.