Is it just me, or does booltool (Union) doesn't actually join Vertices?

Hey guys, i’m newb to 3d printing. so i tried to model something and thought of making it a single mesh by using booltool. but then when i try to use the add-on 3dPrint, and hit Make Manifold, most of my mesh is gone,only a part of it is retained.

I looked into the errors and i see vertices not really joined.

My question is, is there a way to make my object printable without loosing so much detail?
I tried 3d Builder with just clicking Fix It button- but the result looks not smooth.

here:s Original:

here’s Repaired with 3d Builder:

Boolean operations does not join verteces, it joins the mesh wherever it intersect and can create multiple Ngons and lot of problems that in clear geometry had to be usually manually fixed.
For 3D print a have to recommend voxel remesh. You just joint the object by ctrl+J and then set the right amount of detail and remesh it. It will create very dense mesh, after that I usually use decimate modifier and decimate the object even by 80 percent. It will keep the detail where it needs to and decimate verteces where there are no need.

hey bro. thanks for the reply. apparently, i tried to switch to Blender 2.83 version and it(Booleam modifier) did the job for me. because it had the Threshold option. i don’t know why this option was removed in 2.9.

Iam using 2.93 and there is a overlap threshold option.