Are there any websites or youtube videos which are good to learn how to read financial statements when looking at undervalued stocks?

You can google this book call "The Little Book of Valuation" by Damodaran.

YouTube
Dr. Wealth: https://www.drwealth.com/how-to-tell-if-stock-i...

BeattheBush: https://youtu.be/h8MhGY2ivJM

If you want a well analysed website whom does it for you already try https://simplywall.st. They will compare the current price versus what is Fair Value to help you make a decision on if a stock is under-/overvalued.