It's actually very confusing. I think the only good definition is that it's a cultural designation for any company that was focused on digital technology at its inception, which comes with a certain cultural package, and even that has some problems. Netflix is a tech company, not a movie studio, but HBO is not a tech company, even though it also has a streaming platform, and Netflix produces a lot of its own stuff, which is even more confusing because Netflix started as a company that would mail you DVDs. Amazon is a tech company, but WalMart is not, even though Amazon has many physical stores and WalMart does more and more of its business online.

Mechanical engineering can be a part of tech, but again I think it's a cultural designation before anything else at this point. Plenty of mechanical engineers work at Apple, which is definitely a tech company, but if you're a mechanical engineer working on an oil rig, that's not tech.

Add to the confusion that Twitter is a tech company. At this point, what technology is Twitter really developing? Isn't technology about innovation? No doubt that a platform of that size has substantial daily engineering problems to overcome, but like... is that really what we mean when we say technology? Plenty of non-tech companies also deal with the same thing.

I wrote a whole thing fleshing out my theory, if you're curious.

edit: just under this post in my feed is one about how netflix is going to open physical stores.