For example how to align 2 walls perfectly in corner?
you have to have appropriately created millimetrically perfect objects and use the engine’s step movement set to 10 increments or more in order to have any kind of accuracy.
Also corners have to be built special if you have to build lighting
Arent there align button, like aligning edges of object?
No. All engine buttons and inner-working have always worked off of the object pivot point.
There are shortcuts to move it in engine, but I wouldn’t suggest bothering.
You make your object correct in a DCC, import it so the pivot is where you need it, and go from there by zeroing out LOC and moving with the gizmo arrows thereafter.
If your building meshes are setup so the wall and floor segments are some multiple of 100 in width, zero out their location, turn on grid snapping, set that to 100, and move everything into relative place. Then grab the assembled building and move it to it’s intended location. If entire buildings can have some arbitrary non-90° rotation, you could attach all the pieces to a single floor mesh and move that around. This way even if the floor (and thus whole building) is rotated to some weird angle, you can still move and rotate walls and such in local space.
To get the corners of the wall meshes to match, the easiest thing to do is subdivide all the thin edges so the wall has a loop around the center, extrude one end of the wall by half its thickness, then move the extruded corners in by 1mm or less.
The extruded nub will hide inside neighboring walls when snapped end to end and the middle vert will hit the corner of the next wall if its nub is going away(so gotta be careful to rotate walls in the same direction as you build around a floor. In that pic, cube 2 is a linked duplicate. rotated +90 z and placed on center of left edge of floor)). If you edit the normals for all the outer faces to point in cardinal directions, the slightly beveled nub will look flat in-game.
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