In his survey "affine springer fibers and affine Deligne-Lusztig varieties", Goertz gives us a tutorial session on how to use Bruhat Tits buildings to visualize subsets of affine Grassmannians or of the affine flag variety. Due to my own ignorance of the subject, I did not see the precise statements on how exactly points or facets or simplices in a building correspond to closed points in affine Grassmannians or affine flag varieties.

Start with a reductive group G over a local field of equal characteristic, and P a parahoric subgroup scheme, Pappas and Rapoport associated a twisted version of affine flag varieties. Then how can we use the Bruhat-Tits building to think about the points in this affine flag varieties? Even any point to any reference will be highly appreciated!