How Come a PC Needs a Big Stick of RAM To Fit 8GB, But a Phone Can Have The Same Amount in a Much Smaller Space

Technically it doesn't.

Here is a RAM stick for laptop (short) and PC (long).

The black squares are the memory chips, each could be 1GB, or 2, or 4 or so.

That black RAM square is what would go into your phone. And it stays there, never to be moved, until the phone dies. You could solder these squares onto the motherboard of a PC as well. That's the case often in laptops (not always).

But instead they’re soldered onto a separate stick, which goes into a DIMM (SODIMM for laptop) slot, so it's possible to take it out and replace it with a bigger one, or a faster one or so. Desktop computers are built so you can replace things. Anything really. In a laptop the CPU is soldered onto the motherboard. On a desktop you can take it out and replace it. Same with the GPU, heatsinks and fans, power supply unit, whatever.

That's why the RAM is on a large stick.