Musk owns stock worth billions. Stock is a liquid asset, meaning he has billions in liquid assets. The $2.6T market cap is what the public shares of MSFT are "worth." They are owned mostly by people and orgs other than Microsoft, meaning that value is not an asset to Microsoft.

Your comment is low-effort. Don't do that.

Market capitalization means that every share of MSFT combined at the current price is worth $2.6T. The company has $100B cash on hand. That's actual cash and investments that could be easily converted to cash. Substantially less than $2 trillion.

I don't know the first thing about building stairs but I can tell you what I'd do to learn: not use YouTube or social media as a primary source for learning.

You should go to the library and find a book or five that talk about stairs, including basic principles. Maybe buy one of your favorite books or use that information to find one online.

This is major structural work on your home. You can cause a bunch of damage and expose yourself to uninsurable liability if you do it wrong. Unlike many other projects, you can't necessarily redo a wrong turn you take here without substantial investment. If you don't have someone you 100% trust to build stairs themselves watching over you, failure to adequately plan can make this a nightmare.