Question about learning matchups

I looked at a site online, looking for help learning matchups (for ADC) and it mentioned knowing your champion's strengths and weaknesses, analyzing what you bring to the table, what your ally does, what the enemy has, etc. But it mentions things such as "this champion scales" "this champion needs to get picks with their ultimate at level 6", etc. But what can I, me personally, do, to realize things like this. I main Jhin and Vayne but have no clue what their strengths and weaknesses are, what they bring to the table, who their counters (and, the important one, WHY) are, and basically learning what this site explains

The site, i've added as a link, if someone could help me realize what a champion (not just my mains, but ALL champions) strengths/weaknesses/counters, etc. are. Please and thank you for reading!

(I've posted twice today asking a gameplay-based question. If there is a limit or if 2 posts a day is way too many, please let me know)