TSLA is the only way the common joe can invest into other Musk projects. Starlink, boring company, etc etc aren't into the public sphere, TSLA is the default off the added exposures.
If anyone thinks they are getting exposure to the financial gains of SpaceX, X, Nerualink, Boring Company, AIx, Starlink etc. by purchasing shares of Tesal they had better do some reasearch about what they are buying. There may be some collaboration with Tesla but it had better be strictly a formal business relationship if Musk does not want the SEC to be all over him. The only combined companies involved are Starlink and SpaceX.
But don’t take my word for it. Someone should look for themselves. A simple google query makes it simple.