"I can't find ANYTHING on the Internet..." OH REALLY? What are you using as a search term? How cats paint kitchens???
The VERY SIMPLE KEYWORDS, "mona lisa painting technique" pull up MANY MANY articles.
"The first omprehensive evaluation of Leonardo's painting technique on the Mona Lisa started in 2004, and was preformed by the experts of the Centre for ..."
"A look into Da Vinci painting techniques used to create his great works of art."
"S. Lee Hager on focus in Mona Lisa"
"French researchers studied seven of the Louvre Museum's Leonardo da Vinci paintings, including the "Mona Lisa," to analyze the master's technique that gave his works their dreamy quality."
And that was just the FIRST PAGE of results...The articles go into great detail regarding color, value, and paint application.
So, unless you tried searching with keywords totally unrelated to what you are asking us here, there is NO reason you cannot find in-depth and very clear discussion of the techniques used.
You just have to make a bit more effort to find this information on the Internet. I suggest another search, using the keywords presented above...