Which manga chapter does the season 1 of the anime end on? I just finished watching the first season of Nanatsu no Taizai (Seven Deadly Sins). I would like to continue by reading the manga but I am unsure which chapter to start from.

Also, I read online that there is a lot missing in the anime compared to the manga. Is that true?
I read the manga first and started watching the anime halfway through, but the final episode coincides with chapter 100, as far as I can remember.

As far as the manga goes, it has 3 arcs. The first arc ends with chapter 123. It ends kind of abruptly. So if you want to read the manga, starting from after the anime series ends, you have to start with the chapter "Power of Love". I am sure it's above 101, it could be 102 or 103. So starting at 101 would be your best bet.

The anime almost covers what the manga does. Obviously, the manga might feel much granular and in-depth, but the anime gets most of the important point though. Try reading from chapter 101.