regarding IBM, Apple, Microsoft, and Dell: which company now owns part of what other company?

Answer 1

All are independent corporations.

They probably (in some manner) own shares in each other's corps, but if it was significant it would have to appear in SEC filings.

Gates has often supported Apple by contributing $$$ when necessary to keep them in business, so he can point to them as a competitor when confronted with "monopoly" accusations. But this is not a formal arrangement of ownership or partnership.