Creations inside Tiles

Generally speaking, in Scrap Mechanic's Tile Editor you can't directly put blueprint inside a tile.

But it is possible in Scrap Mechanic Mod Tool's Tile Editor for Custom Game. When you edit tiles here, blueprints are referred to the Custom Game ID. Then we have to replace the tile ID with the Custom Game ID. This is to reference the blueprints in the tile, not the blueprints in the Custom Game.

The guide describes multiple ID swaps. In theory this should help when you have already posted a tile in workshop and want to add blueprints to an existing tile. If you change the original UUID of a tile, you will lose the connection between it and the worlds containing this tile.

Careful! Resaving tile in normal Tile Editor will result in loss of blueprints.

If you don't have a tile, create one. Your tiles are located at the path "C:\Users\[YourUser]\AppData\Roaming\Axolot Games\Scrap Mechanic\User\User_[YourSteamID]\Tiles".
Find your tile and open its folder. Open "description.json". "localId" is the tile's UUID. Write it down for the steps 1.6 and 4.1. In tile's "description.json" temporarily change the tile UUID. You may replace the last symbol of the tile UUID with any digit. Tile UUID must be restored to its original UUID after completing paragraph 3. You can find the Mod Tool in your Steam library. In Mod Tool go to File > New Mod > Custom game > Survival Template.
This custom game is temporary. It is necessary to access blueprints in tile editor. Your custom game is located at "C:\Users\[YourUser]\AppData\Roaming\Axolot Games\Scrap Mechanic\User\User_[YourSteamID]\Mods\[CustomGameName]". You also can open it in Mod Tool in Files window: File > Open Folder.

Open "description.json". Replace custom game localId with the original tile UUID (from step 1.2.).

Create folder "Blueprints" in custom game folder ("...\Mods\[CustomGameName]"). This is necessary to put blueprints in it in order to access them in the Tile Editor. Skip this step if you are not using any terrain assets from Workshop.
To use Terrain Assets from Workshop in the tile, you need to copy them in custom game, so they don't disappear.
Dependencies (links to terrain assets) can be found in tile's "description.json":

Go to "...\Mods\[CustomGameName]\Terrain\Database" and open "assetsets.assetdb". At first, assetsets look like this:

For example, let's go with Terrain Assets by Lord Pain. Its fileId is 1331879173. To find it locally, go to Steam > Library > Scrap Mechanic > Properties > Installed Files > Browse. You are now in "...\steamapps\common\Scrap Mechanic". Go to "...\steamapps\workshop\content\387990\1331879173" - that's the folder of Terrain Assets by Lord Pain. Go to "...\1331879173\Database\assetsets.json":

Add assetSetList from "assetsets.json" to "assetsets.assetdb":

From "...\1331879173" copy folders Collision, Database, Mesh, Textures to your custom game folder. Congrats, now you have Terrain Assets by Lord Pain in the Tile Editor!

Your blueprints are stored in "C:\Users\[YourUser]\AppData\Roaming\Axolot Games\Scrap Mechanic\User\User_[YourSteamID]\Blueprints".
Sort folders by date in descending order to find the latest blueprints. Open blueprint folder and copy "blueprint.json" to "...\Mods\[CustomGameName]\Blueprints". Rename "blueprint.json" to "[nameItAsYouWant].blueprint".
Extension .blueprint is necessary to display blueprint in the Tile Editor.

How to make file extensions visible in Windows 10:

1. Click the View tab in File Explorer.
2. Click the box next to File name extensions to see file extensions.

Unfortunately, if you use a blueprint saved in Lift, you will not be able to change the rotation or position in the Tile Editor without snapping to the grid. Even if you change it in object properties and it displays correctly in Tile Editor, it will be broken when you create a world.
To solve this problem and do more flexible rotation and position, you can edit blueprint. For convenient blueprint editing you can make blueprint more readable with the help of json formatter: jsonformatter.curiousconcept.com. As an example, we look at a blueprint that contains several bodies and shapes. Simply put, there are bodies in creation: wheels, doors, etc. Each body contains many blocks and shapes. To move and rotate objects freely in Tile Editor, it is necessary to add parameter "type": 0 at the end of each body.

After editing don't forget to validate your blueprint.

They should work fine, except they don't display in Tile Editor. You can add them similarly to step 1.8.

In Mod Tool go to Editors > Tile In Tile Editor go to File > Open and choose your tile. In Asset Library (bottom left) choose "B" - blueprint For convenient placement you can enable option "Transform around origin" (hotkey [X]). After saving a tile, several differently rotated and positioned creations will have the same rotation and position offset. Only general rotation and position will be saved for both objects and their offset position and rotation will be the same.
If you want more than 1 identical objects with different properties (restrictions, rotation offset, grid position offset), then create different blueprints. Blueprints are not displayed on tiles in World Builder. If there are no landmarks by which you can determine the rotation of the tile, you can draw/build a marker. After completing this guide and creating a world in World Editor (inside the game, not Mod Tool), erase the marker in Mod Tool Tile Editor and resave the tile. In Tile Editor go to File > Save. Save your tile with "Save" option to rewrite it, not with "Save As".

Change the tile UUID back to the original one. It was written down in step 1.2. Copy "Blueprints" folder from custom game folder to the tile folder. If you have completed step 2.4:
Check blueprints, there should be no "type":1 in body if you don't want it to be static and snapped to grid. You need to change it to "type":2 to make it static and free-placed. For the objects that are meant to be dynamic, just leave "type":0. Generate new UUID for it, to avoid possible conflicts with the tile. UUID can be generated here: uuidgenerator.net. Create a world with your tile and see if it works correctly. If you are going to edit tile again, don't forget steps 1.3, 1.6 and 4.1. You can upload your tile to the Workshop in normal Tile Editor. Make sure you don't resave tile here, or it will result in loss of blueprints.
https://steamcommunity.com/sharedfiles/filedetails/?id=3264007062
When you are completely done, you may delete custom game or save it for later.

Thank you for going through this guide!
Let me know if you found any mistakes or inconsistencies.