To give an explanation of the images shown

We have 16 cells on the 4x4 sudoku board, numbered from 1 to 16 from top to bottom and from left to right

taking four cells that do not collide neither in row, nor in column, nor in region we form a template, to place the same digit inside, in this way we generate the following 16 templates

----------------- Cells Number

Template 01 : {01, 07, 10, 16}

Template 02 : {01, 07, 12, 14}

Template 03 : {01, 08, 10, 15}

Template 04 : {01, 08, 11, 14}

Template 05 : {02, 07, 09, 16}

Template 06 : {02, 07, 12, 13}

Template 07 : {02, 08, 09, 15}

Template 08 : {02, 08, 11, 13}

Template 09 : {03, 05, 10, 16}

Template 10 : {03, 05, 12, 14}

Template 11 : {03, 06, 09, 16}

Template 12 : {03, 06, 12, 13}

Template 13 : {04, 05, 10, 15}

Template 14 : {04, 05, 11, 14}

Template 15 : {04, 06, 09, 15}

Template 16 : {04, 06, 11, 13}

Now taking four of those templates and putting them together until filling the board we form the following 12 models

------------ Templates Number

Model 01 : {01, 07, 10, 16}

Model 02 : {01, 07, 12, 14}

Model 03 : {01, 08, 10, 15}

Model 04 : {02, 07, 09, 16}

Model 05 : {02, 08, 09, 15}

Model 06 : {02, 08, 11, 13}

Model 07 : {03, 05, 10, 16}

Model 08 : {03, 05, 12, 14}

Model 09 : {03, 06, 11, 14}

Model 10 : {04, 05, 12, 13}

Model 11 : {04, 06, 09, 15}

Model 12 : {04, 06, 11, 13}

In this way we generate the following graph

https://github.com/MiguelQuinteiro/ImagenesSudoku/blob/master/GrafoCellsTemplatesModels.png