after reading https://www.wikihow.com/Clean-a-Keyboard and other sources I used a 75% isopropyl alcohol solution to clean my black and grey plastic notebook's keys and surface.

I applied the alcohol to cotton pads. Each cotton pad I used came back black, completely black. This is the first time I use isopropyl alcohol to clean this notebook I bought it 8 years ago.

To clean the surface I sometimes pressed the pad with my nails, because I first thought the black residue on the cotton pads was simply dirt and oils accumulated during 8 years. Now, on second thoughts, it could be the isopropyl alcohol solved the upper layer of plastic and now the clearer lines I see where I pressed with my nails are how the notebook looked 8 years ago. Lines look like an anarchic groove, but don't feel different to the finger.

On other parts I can also see the circular motions I applied to clean those parts. Do I have to apply more alcohol or have I already damaged the plastic?

OTOH I cannot believe wikihow and other sources like https://www.howtogeek.com/65073/how-to-thoroughly-clean-your-keyboard-without-breaking-anything/ would include information that hurts the computer.

Am I doing this right?