You can use the Group tool to do this. Select some bricks and then select the Group tool with keyboard shortcut [J] or by clicking on the icon (Folder with arrow in, currently 6th down on the left toolbar). If you want, you can also lock a group, which will "stick" all of the pieces together. If you click on a piece in a locked group, it selects the entire group. This is especially handy for sub-assemblies. If you want to nest your group, make sure that the actual group folder is selected before using the Group tool. Otherwise you will just remove the parts from your first group(s) and place them all in the new group individually.

how do a make costum faces

Here's a tutorial on making decorations. For faces, you can skip step 1.

https://www.mecabricks.com/en/forum/topic/2925