You can select adjacent cells, then merge them into a single cell. Conversely, you can take a large cell that has been created by merging single cells, and divide it back into individual cells.
When you copy cells into a target range containing merged cells, the target range gets unmerged first, then the copied cells are pasted in. If the copied cells are merged cells, they retain their merge state.
Select the adjacent cells.
Choose Format - Merge Cells - Merge Cells. If you choose Format - Merge Cells - Merge and Centre Cells, the cell content will be centred in the merged cell.
Place the cursor in the cell to be split.
Choose Format - Merge Cells - Split Cells.