When I started my author journey, one of the first things I learned about was what an ARC was. It's an important part of most marketing strategies, so this part was easy enough for me to figure out. But then I heard about Beta and Alpha readers and I was confused. So let me break it down for you.
Alpha readers
A trusted person that reads the genre you're writing in. They should focus on the big picture of your story and provide feedback in line with this. They're usually given the rough draft so they're focusing on plot and pacing only, and not grammar.
Beta readers
In this phase, you should aim for more than one person, ideally a handful of people work well. You will give them a more polished version of your manuscript that has gone through a few rounds of self editing. They should focus on the same things as the alpha reader. Because of their focus, it's recommended that your manuscript only goes to a professional editor once you've incorporated their feedback.
It's worth noting that this is a very time intensive exercise for both the author and the reader, so you really need to make sure you're the right fit for each other to get the best results. Some editors have started offering this as a paid service as well.
ARC readers
This is one of the final stages before your book releases, and your ARC readers serve two purposes. The first one is they can help you catch anything that was missed in the previous rounds (usually smaller issues) and their earlier reviews help you market your book.
Having only used the ARC readers in the past, for my next release, The Rule of the Damned, I will be using a combination of both Alpha and Beta readers to see how this adds value to my manuscript. I will definitely do a post in the future about the results and share any learnings I picked up along the way.