By default, entries into unlocked cells in Google Sheets care freeform. You and any editors you share a Sheet with can type or paste any numbers, letters or symbols into an unlocked cell.
Even images can be entered into Google Sheets cells.
This is one of the aspects of spreadsheets that makes them different from database applications — a lack of rigor.
But it is possible to enforce the entry of only certain values in Google Sheets cells with by creating an in-cell dropdown list.