This gives you the right “to beat them up” but not the right to learn a secret. You can take a patent and build that thing in your house. The government can’t stop you, neither the inventor. It’s when you try to sell it that they can come after you.
I don't think that is correct. The patent act states:
>...Except as otherwise provided in this title, whoever without authority makes, uses, offers to sell, or sells any patented invention, within the United States or imports into the United States any patented invention during the term of the patent therefor, infringes the patent.
https://codes.findlaw.com/us/title-35-patents/35-usc-sect-27...
Note where it says "makes, uses". In practice, it is highly unlikely that someone would know about the infringement if it was just done for personal use, but that doesn't mean it isn't infringement.
It is. It's often called the "research exemption." You literally can only get into any kind of trouble when your intentions are commercial.