This instruction just illustrates the process of a simplified model
Step one, delete the cube🤡
Make sure the Image as planes addon is enabled and add images as planes (Import -> Images as Planes)
Create your model, add the necessary materials (texture image) like here the bark for instance:
in that case the bark should be UV unwrapped and then scale it until it fits:
Select all image planes and duplicate them (shift+D), then flip the duplicated faces (Mesh -> Normals -> Flip):
Select the joined object and make a new UV map (important that the new map is selected, but not necessarily the camera view (symbol) must be enabled): 
Join all the objects to one, Ctrl + J:
Make a new texture for the baked image:
Make a smart UV Project of the joined model:
Add the nodes for the new baked texture (for any of your materials!):
setup up the render settings:
Before you bake make sure all objects are selected and the right UV map too! Set type to diffuse and deselect the light, then bake:
Save the image of the baked texture:
Then remove all the materials (because you just want one single one), add a fake user if you dont want them to disappear:
Then make a new material which serves as a slot for the texture you wanna add it to later, map the new texture to it:
Make sure that right settings are selected, if you have more than one collection it may export all of them:
Blender will use the first UV map for the exported model, I suggest to remove all older maps to not get in conflict with them:
the settings of the model in the game engine:
and the settings of the texture:
and on the gameobject (if you have the same retarded Engine like I have, Unity, sigh):
