Convert meshes to single smooth surface

Hello! I have a problem converting complex meshes into a single smooth surface. I tried many different options but all were unsuccessful. I also converted the meshes into a polysurface trying to get to my result but the file was too big. At least I was abIe to attach the meshes file. If anyone can help me I would be very very grateful!!! It’s a terrain modeI which I need for an architecture project.
Best regards

3dmodel_meshes.3dm (7.2 MB)

Select all your meshes, run “_Join” command, then run “_Weld” command.

Hello - to you need a surface (as distinct from a mesh or even a SubD object) in the end? What is the end use of the object, in modeling terms?