How do I pick an object using its name?

You would just pick by the variable.

Sprite is called Sprite.

If you pick by the object you pick all the instances, or the first instance depending on the action.

Sprite set angle to 90

If you pick by the object instance variable you "filter" out all the instances that don't match and pick all that do.

Sprite variable myvariable = 1 set x to sprite.x +1

The variable isn't the only way to pick, but it is a standard one.

Instance variables are located in the preferences side after you have selected an instance in the editor, or from the object list on the right.

The starting tutorials should show that very well.

Perhaps I didn't make myself clear.

If I have a sprites called 'floor', 'wall', 'ceiling' etc., how would I choose one of them using their name?

The objects don't have a expression for their name that is a string.

If you want to use a string to do that then you would have to use a function map.