From Minetest

Building—also called placing—is the activity of placing blocks next to other blocks. It is one of the most basic things you can do in Minetest.

Building requires the “interact” privilege (a default privilege).

To build, one must wield something which can be built, point to something which can be built to and press the build key (Right mouse button by default). Most blocks can be built. Building is, unlike mining, always instantly done. On blocks which can be used (e.g. Chest, Furnace, Sign, etc.), you have to press Sneak+Right-click instead.

Minetest Game

By building a slab onto another slab of the same material, a full block of the same material is created. It is possible to build inside some nodes which entirely destroys them. Air, can always be replaced, as well as liquids like water, too. Many plants like grass can also be built inside.

For Tree, Acacia Tree, etc. and Cactus, the initial facing direction will be from the block face you placed the block on. E.g. if you placed a tree on the floor, it will stand vertically, and if you placed it at the side of a block, it will be horizontal. By holding down the Sneak key while building, you can reverse the placement behaviour: Placing a block vertically rotates it horizontally, and vice-versa.

See also