I didn't understand very well this part, since English is not my native language.
LATEXMK *vimtex-compiler-latexmk*
As stated, one may customize the latexmk options through
|g:vimtex_compiler_latexmk|. However, one may also configure latexmk
explicitly through a global ~/.latexmkrc file, or a project specific
.latexmkrc file. It is important to know that command line arguments have
priority, so one may want to use custom options if one wants to specify
particular things in a configuration file.
I do have a latexmkrc file with my settings. In .vimrc I have only
let g:vimtex_view_method = 'zathura'
let g:vimtex_compiler_method = 'latexmk'
So every time I use <leader>ll will use my config file, or some other default config for latexmk hidden in the plugin's code?