Two things to know: the browser asks permission before a page can read or write the system clipboard, so the first copy or paste may prompt; and a read-only cell is never a paste target, so pasting onto one changes nothing.
Drag across a block of cells, or extend it with Shift and the arrow keys, then press Ctrl+C (Cmd+C on a Mac). The grid puts the block on the clipboard as tab-separated text, the same form Excel, Numbers and Sheets all read as columns and rows, and the box below shows exactly what was copied. Copy a block from a real spreadsheet and press Ctrl+V over a cell here and it pastes in, cell by cell, through each column's own type. Read the full how-to on latticegrid.dev.
Two things to know: the browser asks permission before a page can read or write the system clipboard, so the first copy or paste may prompt; and a read-only cell is never a paste target, so pasting onto one changes nothing.