There is no delete key on the standard Android keyboard to delete in front of the cursor. You either have to switch to an alternative app or you activate the function of the arrow keys for the space bar, and this is how it works:
> Settings > System > Language & input > Onscreen keyboard > Gboard > Glide Typing
Enable glide typing
Enable cursor control by movement
Now the cursor can be moved precisely in the desired direction by swiping on the space bar in order to be able to delete the desired letter.