I don't know the entire history but sometime back in the 1990s when I was young and stock commissions high, I bought several stocks as DRIPs (dividend reinvestment plans where you could reinvest dividends and buy more stock w/o paying any commissions) and one was Upjohn. That ended up being a rather confusing mess of acquisitions, mergers, spin offs. I think it included the following:

1995 Pharmacia & Upjohn was formed through the merger of Pharmacia AB and The Upjohn Company
2000 Pharmacia & Upjohn completed a merger with Monsanto and Searle creating Pharmacia
2002 Pharmacia completed the spin-off of its agricultural subsidiary, Monsanto Company.
2003 Pfizer Inc and Pharmacia Corporation began operating as a unified company
2020 Pfizer spun off its Upjohn unit which immediately merged with Mylan to form Viatris

Fortunately I never ended up with a huge amount and anything I sell I will use a rather low cost basis since I'm not sure I have kept all of the details above over the last 25+ years. I think in the past I used something close to a zero cost basis just to avoid tax issues.

And yeah, sometimes things get too big and need to break apart and start over.