In 2015 TT Games maked Lego Worlds - open world game voxel-like game. It runs on 2012 year laptop without lags in 720p.

So, here the questions.

Your assumption.

1. How they store that amount of data?

2. How they draw lego elements (in landscape, for example). Is it 3D models of bricks or procedurally generated meshes? Is a face culling there?

3. Do they use simplified mesh for LODs?

4. What a type of algorithms used for that type of placing bricks?

Thanks.

We don't ever know exactly how they did it unless they publish some blog posts or development papers about it but

chrun said:
How they store that amount of data

It is less data as you might think, I can imagine a few bytes per brick which are location, transform, color and ID, maybe 9 * 4 = 36 bytes per brick.

The map itself is a hightmap with some biome data like in Minecraft

chrun said:
How they draw lego elements

Like you draw every other mesh. In a far view LODs are used and if you come closer, they turn into more detailed meshes until you are really close. I can imagine that the pins on every brick are just bump-mapped like you do for wall details in modern 3D games.

A bump-map is just a texture that gives additional hints to the lighting system to proper illuminate small details without the need of an artist to model those details into the mesh.

chrun said:
Do they use simplified mesh for LODs

Every game uses LODs these days to be able to maximize performance and render depth.

chrun said:
What a type of algorithms used for that type of placing bricks?

As I said in the opening sentence, I don't know because they didn't published any details so far but you can read about how Minecraft and other Voxel-Type games solved their problems and we also have already long discussions here on GD.net about the topic