Sweet game! Gave me heavy mid-2000's flash games vibe, with the gameplay, heavy linework, etc. And for that reason I like it since I remember really liking those physics bridge building games back then. This is so feature-packed too, every time I thought I had seen everything you introduced a new mechanic!
I got to the level with 25 springs and glass balls and didn't continue past then because it was just tedious to place all of the springs again whenever I had to reset.
If you develop this game further, I suggest that you make it so it saves the current design that the player has when they fail and just allow them to delete connectors and place them again. This would encourage more experimenting and make it less frustrating when the player fails on one of the levels where they have to place 20 connectors (or maybe just limit the amount of connectors per level to like 7-8, either works)
There's also the small issue that it's sometimes hard to tell what is in the background and what is not (this first came to be an issue in the first level with the yellow metal)
Overall though, great polished game, I had a lot of fun with it!