Do power outlet splitters decrease power for each deviced connected to them?
If split, all plugs will get the same access to "power".

Your outlets themselves are splitters: in general several outlets share a run of wire back to the power source.  The external "spliter" is no different, it just provides more connection points to the same electrical circuit.

However: the total circuit capacity is neither increased nor decreased by a splitter. Devices "draw" current based on their needs: for example a motor generally draws more power when loaded than when spinning freely.  If the total draw exceeds the circuit capacity, a protective device will trip.  The protective device is usually set at 15 or 20 amps in a residential circuit.

Extra connections or total extension cord length can have a minor effect: a long thin cord with a heavy load can heat up, using up some of the electricity in transmission. In effect the wire becomes an electric resistance heater.  This effect is usually negligible.