These are platform capabilities, so they behave the same way in every product rather than being configured separately in each.
Switching product in Treepie does not mean switching application. The navigation, the search box and the saved views are the same in Flow as in Books, and moving between them keeps you on the record you were looking at rather than dropping you on a dashboard.
A search for a customer returns the deal, the project, the open tickets and the unpaid invoices together, because they are facets of one record rather than four documents that happen to share a name. This is the part that is impossible to retrofit onto a stack of integrated tools: they can sync data, but they cannot share an index of something they each define differently.
A view you build in one product behaves the same way in another — same filters, same columns, same sharing. Bulk actions apply the same permission checks as single edits, which sounds obvious and is the thing most systems get wrong.
No. One account, one workspace, and the products you have switched on appear in the same navigation.
Yes. Product access and record-level permissions are separate questions, so somebody can have full access to Desk and no visibility of Books.