Backend process of lmer function I am trying to learn mixed models using lmer function and I am able to use it. But I am not able to understand what is going on in the backend like how it is taking care of fixed effects and random effects and the statistics behind it. All the source which I referred only goes through the syntax and not the backend process. It would be great if anyone could help me understand the process or suggest me a source which I can use for reference.

Check the paper by its authors that goes both through the math and implementational details:

Bates, D., Mächler, M., Bolker, B., & Walker, S. (2015). Fitting Linear Mixed-Effects Models Using lme4. Journal of Statistical Software, 67(1), 1–48. https://doi.org/10.18637/jss.v067.i01

