Right, I try to extract the main information for you. As a "normal" road spline he uses RQ10,5/7,5 2-Spur (so two tracks) from the Simple Streets pack and Folti's tunnel spline. Links for downloading them are in the description.
At 1:10 he claims that he takes Folti's tunnel spline as it uses collision mesh, which is not used in the Simple Streets Splines.
1:30 Three splines, tunnel spline is attached behind it
1:55 the spline that goes into the mountain is set to "Yes, no THS" at Align Terrain.
2:15 same with the one directly before the tunnel spline
2:38 the tunnel spline itself is set to Yes, both THS -> save map
2:55 he contradicts his 2:38 statement and sets the tunnel spline to Yes, no THS and moves it around.
3:10 He sets the first spline to 10 meters and the next to 100m, tries out 1, 2 and 4 meters for the first tunnel spline for getting rid of terrain cutting through the top of the tunnel
4:40 applied cosmetics
5:20 same principle when the tunnel comes out of the mountain again
Hope I could help. Have fun with your map and your tunnel.