Microsoft briefly overtakes Apple as most valuable US company

No, they didn't.

https://www.nasdaq.com/symbol/aapl https://www.nasdaq.com/symbol/msft

A slew of articles like that have been coming out because Google's data is wrong.

It appears this might have been a reporting mistake: https://9to5mac.com/2018/11/24/apple-market-cap-microsoft/

Microsoft is definitely more diversified than the other big tech companies.