Project Manager is INV_INVESTMENTS.MANAGER_ID (which is a lookup to a resource) - i.e. the MANAGER attribute on the PROJECT object.
But I don't know what your "Project Summary Page" is, so its possible (maybe unlikely) that it is some custom field - if you needed to trace where that comes from you should start with what is being displayed on the page and work back to its source I would think (rather than starting at the object and trying to guess).
The data-model is documented to a limited extent in the system technical reference documentation (including ERDs for some areas), but there is no "all attributes everywhere" listing as far as I know. You could query the database itself for columns but not all application "attributes" map simply to database columns - it can get quite complicated in some areas.