Wiring two receptacles to an existing one
Normally when there is a switched receptacle only 1 of the 2 are switched. The cable that is black , white, red &amp; ground.
the white is neutral Silver screw . the red goes to the switched hot Brass in color.
the tab between the top and bottom is broken on the hot side or the side that has the red and black wire.
The black wire normally always hot Is on the other brass connector.  The second cable is usually black to black , white to white and bare copper to bare copper.
This is how it would be wired when the power is going to the switch from the panel then the extra cable black white feeds another receptacle, to add another non switched receptacle tie to the black white ground.
If the room doesn’t have a light fixture controlled by a switch you need to keep the switched receptacle for code compliance.