If you want to learn something, you have to be first and foremost willing to practice today almost forgotten activity - reading.
The wheel bearings have a standardized dimension / size for most of the wheels in the market. These days (= summer 2018) a new size of the wheel bearings has come to light, the so-called 12 standard. The 12 STD bearings (and the wheels too) are bigger compared to regular bearings. The axle is 12 mm in diameter.
The vast majority of the wheels come with pre-installed bearings.
When replacing the bearings it is necessary to get the old bearings out and get the new bearings in.
With plastic wheels (cores) that job is easy. All you need is one finger to push the old bearings out of the core and then get the new bearings in.
With aluminium cores the job is a bit more difficult. One finger strategy would not work anymore. See the following video to learn how to replace the bearings and spacers https://www.youtube.com/watch?v=W4NKGBfa1Po
The vast majority of the wheel bearings today are supplied with the so-called spacers (washers) which are absolutely necessary for the proper functioning of the bearings. Without the spacers, after the wheel has been mounted and tightened, the bearings would squeeze and would not spin properly. It is always a good idea to keep the original spacers (see the "Scooter wheels" chapter)
What is ABEC?
It says almost nothing about the impact resistance. The values ABEC 9, ABEC 11, etc. indicate the accuracy of the production. The higher the number, the more precise the bearing is, the smaller the friction on the inside of the bearing = you get faster bearing. However, that is not to say that ABEC9 bearings will be more durable / will last longer than ABEC7 bearings.
The type of bearing
Any bearing is covered with a dust cover. Plastic cover on one side = RS bearing. Plastic covers on both sides = 2RS bearing. Most bearings have an outer diameter of 22 mm and an axle diameter of 8 mm = type 608. 12 STD bearings have an outer diameter of 28 mm and an axle diameter of 12 mm = type 6001.
How to extend the lifespan of the bearings
a) No water = no riding in the rain or on the wet surface. Humidity makes no good.
b) No dirt = do not ride on dusty roads / spots
How to select the right bearings
Avoid buying your bearings at inline skate shops. Inline skates have four times more wheels and are designed for fast, smooth ride on a flat & straight track. They are not built to withstand the big impacts coming from riding a scooter. The ABEC11 ceramic bearings, which cost a fortune, can easily fall apart after the first jump.
It is recommended that you get your bearings from the same brand as your wheels. Ethic to Ethic, etc. Issue being the spacers that might slightly differ brand to brand.