Keeping related data in separate tabs of one file, instead of separate files entirely, is what multiple worksheets are for.
The + button next to the sheet tabs adds a new one. Double-click a tab to rename it. Click a tab to switch to it.
=Sheet2!A1 pulls the value from cell A1 on a different sheet — the sheet name, an exclamation mark, then the cell reference.
Click one sheet tab, then Ctrl+click others to select several at once — any formatting or entry then applies to all selected sheets simultaneously. Useful for building several identical-layout sheets (one per month, one per branch) at the same time.