What did tunnel digging teach us about the formation of chains of mountains?
Tunnels have provided nothing more than what was learned from conventional geological drilling and surface mapping. Tunnels provide supplemental information. Very rarely do they provide "make or break" information that is pivotal to geological understanding on a large scale.

A tunnel is a lateral large diameter hole. Geological drilling produces small diameter holes, whose orientation can be lateral, vertical or anything in between. A sample of rock is take by both.

By using in-hole surveys and noting from where, within a hole, a sample was taken from it is possible to know where the rock sample came from. This is useful for creating geological maps of the region. It is even possible to place ground stress measuring devices in both drill holes and tunnels.

The mass of material taken from a hole, whether it be a geological drill hole or a tunnel is not particularly significant in expanding knowledge. More knowledge can be gained from multiple small samples taken over a large area.

Tunnels do give geologists the advantage of walking through a rock mass and seeing some structures like faults. But if the tunnel hasn't intersected a structure then the geologists may not know about it.

Your question is a bit like asking "what can be learn about a country by just walking along its roads". Additional information can be gained by walking off track, navigating rivers, flying over the country and talking to the inhabitants.

Tunnels provide some information. It is useful information, but multiple sources of information given us a better understanding.