Here's the break down and my experiences with them.
Alpha: as the word entails, this is your first ever reader of a rough draft. They are to tell you their likes and dislikes, what works or doesn't. Usually, you should select someone close who can be honest but give you a kind delivery of criticism. They look for continuity issues, major plot holes, point out unanswered questions and such.
I don't have an alpha, or more like I only use a beta, meaning I do not send it out to anyone until it is revised and has a quick edit. I put it away and come back with fresh eyes and critique it myself making notes of what doesn't work, inconsistencies, character issues, etc. This is hard to do in an unbiased way, but I am my harshest critic, so I tend to do pretty well. However, I doubt anyone in the publishing world would suggest what I do. I have degrees in literature, teach grammar, am well read, and spent my life analyzing and teaching texts. Although I'm far from perfect, and cannot see my own mistakes, most of them are at a sentence level or my beta finds any global issues.
Beta: after you've revised and edited at least once, you move to a beta reader which is someone who reads your book from the perspective of your average reader. Like the alpha, they point out strengths and weakness in a supportive way. They are the second line of defense. After that, you revise and edit again and hope it is ready for querying.
I've got a friend who does this, but she goes overboard and is my first editor who is aware of my weaknesses (dialogue tags, word repetition, mixing up names, etc.). She's candid, helpful, and is an avid reader who knows what she likes and what does well in most of the genres I write. She also brings more out of me. Asks me questions, tells me what she wants to see, asks me to build my world and details more. She basically helps me get what is in my imagination onto the page because not everything makes it the first couple goes. My mom reads it as well and tells me what she liked. Since moms can't be unbiased, I read between the lines--whatever she doesn't rave about, I try to make stronger.
Most professionals will tell you to use between 3-10 betas. I have done that twice to my detriment and trying to undo the changes I made from their suggestions. A lot of betas is not bad advice, but I have the inability to weigh what they say and want to please everyone. Neither time did I get a consensus of advice, nothing lined up, and my head spun. I used the pattern above of just my two betas and those two got publishing deals, the other two did not. But this is probably just me. Maybe next time, I'll try three.
Sensitivity: like a alpha or beta, a sensitivity reader is reading and looking for things that work or do not work but through a particular lens. They will make sure that you are representing a group of people unlike yourself fairly without stereotyping or being offensive--a different race, sexual orientation, or if you have disabled characters or discuss a controversial issue you are worried could be taken the wrong way.
I have only recently embarked on writing a contemporary romance with social issues. I have only just started but already questioning myself. I'll be dealing with issues I have a lot of first-hand experience with but from an outsider's perspective, meaning I'm not diagnosed with the disabilities I wish to write about. You better believe I'm having a sensitivity reader go through it. Recent events where an author pulled her 6-figure-deal novel from publication for revisions due to test readers being upset by depictions of slavery is a stark reminder of the sensitive world we live in, and no one wants bad reviews or to upset people.
So the question is, do you need all these types of readers? My opinion is it can't hurt. And if you are planning to self publish, I'd say the more the merrier because you don't get all the filtering and editing publishers do for the traditionally published authors. Betas can be your line of defense. I also suggest arming yourself with these readers if you've gotten a lot of requests for full manuscripts, but no offers. There might be something you're overlooking only an outside reader can see. Basically, it can't hurt as long as you're NOT like me and take everyone's advice. Weigh things carefully. But hey, lesson learned in the end.