I have made a few PCB's over the past few months as I have started to learn about design. All of the circuit boards have been Arduino based, so nothing to powerful. I design them up in Eagle and I know this is bad, but I have just used the Auto-router for everything, including the ground and power lines.
My boards have worked fine with this (maybe except for one thing), but I know this is very bad practice. I have been doing some research on ground planes and from what I have read I have not found a reason not to use them on your 2 layer PCB. Should I always add a ground plane to the bottom of my circuit board? When I look at other example boards in Eagle like the Arudino Mega, I notice it does not have a ground plane and I would like to know why someone would not use one. Also, is it okay to run signal wires through your ground plane?
One other question I have is when is it okay to use the Auto-router. From what I found online it should be fine to auto-route signal wires, and that I should manually route ground and power traces. And that these traces should be much thicker than the signal traces.
My last question is about ground loops. I think I have paranoid myself about ground loops and think that they are always gonna happen. As long as I have everything attached to ground on my board (and I only have one common ground), then there should be no issue in having a ground loop correct?
Thank you for the help, I am still very much a beginner with all of this.