Say a large business such as Apple buys 10% of CitiBank. Is that public information? If so, where can that information be found?
In the US, shareholders owning 5% or more of a public company must file a "beneficial ownership" report. In addition, transactions involving shareholders holding 10% or more must be disclosed. All these can be found in the SEC database. See here for details.
These are SEC rules, so they only apply to companies listed as publicly traded in the US. In other countries rules may be different.