Confused About the Usage of Korean Particles 서, 에서, 아서, and 어서!

I recently started studying Korean again, and I'm a bit confused about some grammar elements, so I would like to ask for help. I learned in Chapter 3 of the TTMIK book that '그래서' means 'therefore' or 'so,' but I recently heard that '아서' or '어서' are used more frequently. If both have the same meaning, I'm curious which one is more commonly used. Also, how should I distinguish between '아서' and '어서'? As far as I know, it seems to depend on the last vowel, but I'm not sure and would like to confirm. Additionally, I'm confused about the difference between '서' and '에서.' At first, I learned that '서' indicates a location in sentences like '여기서 기다려,' but later I heard that '서' is not used alone and has a meaning almost identical to '에서.' Could you help me understand these two in comparison? I would appreciate any clear explanations or advice regarding these issues!