Where is the starter relay switch located?

It is in the battery junction box under the hood. Testing is simple, swap it with the blower relay. If it now starts, it's the relay. If not, then it is elsewhere in the system. What is the issue you are having?

Starter relay should be the ice cube style in the outer corner. Before you condemn the starter, you may want to test it on the vehicle. For that you can just use a short chunk of wire to go from the battery positive post on the starter to the starter engage post. That is the smaller wire that connects to the small post on the starter solenoid either bolt on or a push on depending on how they put it together. I would also use a set of jumper cables with the positive going from battery positive to the positive post on the battery and negative going from battery negative to a good ground on the engine. Now use the short wire to jump between the positive post and the starter engage post circled in image. Does the starter now run? If yes it isn't the starter. With the jumpers connected try the key, Nothing? Try shifting into neutral with your foot on the brake try the key? Now let's say it does turn over when jumped and does with the jumper cables attached, disconnect the jumper on the ground side and try it what happens? Now try it with the positive side disconnected at both ends anything. Those tests are to isolate where it has an issue. If it turns over when jumped, that means the starter is okay, if it doesn't turn over when you disconnect the ground side then there is an issue with the ground cables or connections. If it doesn't turn over with the positive side disconnected, then that cable is suspect. If it turns over with all the above but not when you use just the vehicles systems, then we have more testing. Most of which can be done with just a test light. So, get a test light, connect it to battery ground and we can start at fuse 9 in the under-hood box, it should have power at both sides with the key off. Now go to fuse 21 in the fuse box on the driver's side. With the key held in the start position it should get power to both sides. That tests the ignition switch and its wiring. Next go to the relay and remove it and probe the 4 contacts, you should find power on one pin. Now have the key held in start, you should now have power on two pins. Check that power in both park and neutral. If both work, you know the neutral switch and its wiring are okay. Now connect the test light to battery positive and probe the two pins that did not have power. One of them should light the light and may engage the starter as it should find a ground through the starter solenoid. If it does you know the starter control wiring is okay. The other should ground through the ECM. I suspect that one of these tests will show up as bad. That will show you where to look next.

Okay, if it sparked but didn't turn over it could be a bad starter, that connection only activates the solenoid that has a high-power switch in it that actually connects the battery to the starter. With the number of bad out of the box parts these days anything can happen.
So, you swapped the relay and the starter and tried jumping the starter, have you had the battery tested just to rule it out?

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There shouldn't be a ground wire on the starter, it grounds through the engine block. There should be a large red line from the battery to the top solenoid stud. Then a Brown wire with green stripe on the start terminal. The other cable on the solenoid should go into the starter body itself.