How to install R 3.6.3 (NOT 4+) on Linux MINT 19.x (19.1, 19.2, 19.3)

tl;dr On Linux MINT 19.2, I was only able to properly install R 3.6.3 (but NOT R 4+) The correct repos need to be updated in at least 2 files, only then can R be installed Did I miss any tips? please leave me a comment. Background I have tried, and failed, to …