Why do my granny squares look lopsided once I join them?

Your instinct about tension is right, that's ussually the main culprit. When you're joining, you're holding a hook and working along a raw edge, and most people naturally grip tighter because the squares feel floppy compared to working in the round. Every stitch gets a bit shorter and tighter than the stitches inside the square, so the joining row acts like a drawstring and pulls everythign in, wich is exactly what makes edges ripple and corners poke out. Two things that helped me: go up a hook size for the joining round only, and consciously relax your grip. Some people also work joins with the squares flat on the table instead of held up in the air, which stops you from unconsciously dragging them out of shape.

As for blocking, you don't need mats and pins and the whole setup for acrylic. Steam blocking is your friend here. You can literally pin the squares out on a folded towel on your bed or the floor, hover an iron above them (don't touch, acrylic melts) or use a steamer, and let the steam relax the fabric while it cools. Takes a couple minutes per square and it makes a huge difference when you go to join, because suddenly every edge is the same length and lying flat instead of curling. Do I pin every square? No, honestly. I steam them, stack them, and count stitches at the join. What matters more than pinning is having the same number of stitches along each edge, so keep counting, that part you're doing right. On the oversized squares, don't frog them. If one's only a bit bigger, steam it harder and pin it down while it cools, that usually shrinks a granny square a surprising amount. If it's still out of line, you can cheat at the join by working two stitches togather into one stitch at intervals along that edge, which eats up the extra length. Frogging is really only worth it if a square is wildly off, like a whole round too big. Side-eyeing ten squares is normal, by the way, everyone's first few squares in a project are tighter than the last ones.

I remember when I was putting together big throws back in the day, the biggest thing was making sure the joins themselves looked deliberate, not just shoved together. If you're worried about those corners poking out, sometimes the issue is that the join stitch doesn't lie flat against the finished border of the square. Try using a single slip stitch for the perimeter, which holds the fabric a bit cleaner than a single crochet border. It creates a tighter, flatter seam that is less likely to distort when the squares are placed side by side. Also, if you are using a bulky yarn, you might consider even swapping out the join stitch entirely for a very tight double crochet to make sure the new line anchors properly to the existing fabric.

I started this big blanket project a while ago and it feels like I spent most of my time making the individual pieces instead of acutally making the blanket. Every time I finished one square, I wondered if I was doing it right before moving on. The yarn itself seems to be stretching a little when I work it. I guess when you're doing this kind of work, there is a specific way to tell if a square is totally done and ready to be sewn up. How do people usually know that a square is finished?

Update: My local shop said the tension was the problem, so I made sure to keep my hands loose while joining the squares. It helped alot with the rippling, though the corners are still kinda jutting out. Do you all find that a specific join stitch makes the seam lie flatter against the finished piece?