This is a situation where the vocabulary becomes particularly important. A "sheet" in Numbers is a "tab" within a document. Each sheet can contain one or more tables, and the tables contain cells.
Yes, a formula can refer to a cell in a table on another Numbers "sheet" in the same document. But a formula cannot refer to a cell in a table on a sheet in another document.
That is rarely needed, however, since Numbers makes it so easy to have multiple tables on one sheet.
Numbers is not web-centric like Google Sheets. You can place hyperlinks in cells that call up a webpages or emails. But you can't link between cells or documents or apps...except by script, which is often more trouble than it's worth.
SG