Cable bought on long spools to install permanently is usually solid, while patch cables are usually stranded. Most of the time, you'll end up buying pre-terminated stranded cable, and all the cable you terminate yourself will be solid.
Does it even matter if the cable is solid or stranded? When you put on that new RJ45 connector the cables go into the connector with the insulation on.
I tried and failed three times. Gave up, and pulled solid cable. TIL that some rj45 connectors have multiple teeth, and interlock.