Here I will post many corners of the Internet which I found useful in one way or another.
Causal Inference: The Mixtape by Scott Cunningham
Program evalutation for Public Service by Andrew Heiss - This is probably one of the most amazing resources for learning about modern problem evalutation with videos, exercises, worked through and illustrated modern R code examples!
(Great) lecture notes by Clement de Chaisemartin for PhD students
A very thorough overview with many of the variants in the recent literature
Good overview of the recent literature on DD and Two-Way Fixed Effects by Andrew Baker