They had vertical integration and made bank. Then each layer got more complex as computing became mainstream, and IBM got out-competed on everyone of them by specialized companies. And then they sold them off. The only layer that remains is the most strategic: the customer relationship.

IBM is now just a consultancy.

They became NSC, no such company.

Sometime in the 90s they decided that becoming a services and consultancy provider was more important than being involved in large-scale commodity hardware or really anything else.

Over three decades they've exited commodity hardware (x-series and thinkpads), retreated from a lot of software stuff and generally gone as hard as they can for large-scale business contracts. They seem to keep a decent amount of cutting-edge hardware research going, Power is still broadly competitive with x86 AFAICT, but the volumes have got to be pretty low these days and I half suspect that some of it is just a "Hey look at us we're still here" play rather than actually feeding into products, because services are the profit-centre.

They're still huge and have a lot of money, and they own Redhat. But IMHO it's in a long, slow decline, and it has stepped back from the public eye and most public-facing markets.

Context - I've worked for them in a few places. Obvs my opinons are my own, especially as I've been 'out' for over a decade at this point.