Blender instances to prefab ?

Is there a way to export blender instances into prefabs for unity ? Like a blender add on or something like that.

I have a huge mesh created using geo nodes with blender that uses 4 differents instances and i would like to have them into separate prefab

From what I am understanding, you have a blender model in four parts and want them as separate prefabs. What I would do is (after you have the model in your assets folder or wherever you may keep it) drag it into the scene, unpack it, delete the other three parts, drag the model with only one part from your scene back into the folder to make it a prefab again, rinse and repeat for all four parts (drag in the original model and unpack every time!).

Without seeing the blend file its difficult to recommend, but is it easier to join them in blender? Select all the same tree and press CRTL+J

Select the new joint object and when exporting tick the “Selected Objects” that will only export the trees selected, do that for each type of tree and then you will have four separate objects in Unity.