I use this feature all the time…
Having opened your sheet on Android, simply tap the cell you wish to access
and at the bottom of the screen you will see the place where you can Enter text or formula. Once done, tap the tick ✓ (right of screen at the end of the entry panel)
and then tap the tick ✓ at the top left hand side of the screen and the job is done.
You can only enter data in one cell at at time but if you are wishing to select a number of cells (for example to edit colours, formatting cell ranges etc), use the corners of the cell selected and drag to open a range (but you cannot enter data that way nor can you, for that matter, in a browser. Neither can you you cannot select multiple unconnected cells.