The best way to learn about threads is to experiment with lots of different kinds. But some basics about thread weights and sizes are the higher the number the finer the thread. 100 wt silk is super fine and there are 100 wt polyesters too like Superior Microquilter and Wonderfil's Invisifil. The lower the weight number the heavier the thread. So an 8 wt is usually a perle cotton most commonly used for hand work. It is way too heavy to run through the top of any machine but can be wound onto a bobbin to do bobbin work. The second number, if given or applicable indicates how many plys are in the thread. So in your case of aurifil 50/2 that means the thread is a 50 wt but only 2 plys. So it is finer than a 50/3 cotton but also weaker.
I will often use numerous types and weights of threads in a single quilt. My go to piecing combination is aurifil 50/2 in the top and Superior bottom line poly which is 60wt 2 ply in the bobbin. I only piece with a single color. I have a light beige/taupe for lights and a dark charcoal for darks. If I am piecing a light to a dark I stick with my light neutral combo. As long as your tension is good and your stitch length short enough, the light thread used in piecing doesn't show. My friend who makes many top winning show quilts pieces exclusively with Wonderfil Deco Bob which is an 80 wt 2 ply. It is super fine and her peicing is impeccable and her blocks are perfectly flat because the thread is taking up so little space in the seam.
When it comes to quilting the sky is the limit. Don't limit yourself to only a few types/weights of threads. Consider what look you are going for and what the application is as well as what kind of quilt (a utility quilt that will be used a lot and laundered a lot as opposed to a quilt that won't be so heavily used or a wall hanging). For example of application; for SITD (stitch in the ditch) I may use a monofilament thread like Superior's Monopoly (a polyester) or Glide's Essence (a nylon) or I may use Invisifil. These threads are super fine and slipping out of the ditch is camouflaged because the thread is so fine. If I am doing a very dense background fill I turn to Invisifil again. Because this thread is so fine the buildup of back tracking doesn't show and it sinks right down into the fabric and mimics the color of the fabric or just adds a subtle hint of color if you are using a contrasting thread color.
For a lot of my general quilting I use a 40 wt trilobal polyester. Mostly Glide brand but I have also used Isacord and Metro Embroidery thread. These are all 40 wt trilobal polys and are very shiny and strong. I quilt on longarm so I normally don't use cotton for quilting just because of all the lint. But if the color/look is just right for the quilt I won't hesitate to use a cotton. I have used YLI, King Tut and Connecting threads cotton. They all worked just fine for me but I did find the King Tut to produce the least amount of lint. In my LA bobbin I use, almost exclusively, magnaglide prewound bobbins. Magnaglide comes in two weights. A 60 wt is in their bulk boxes and comes in limited color selection. The magnaglide delights are sold in little jars of 10 bobbins and loaded with the 40wt. glide. I will run the magnaglide with any kind of thread in the top.
Then there are all the fancy decorative threads, many of which can be run through a longarm. They can be much fussier but can really add a lot of visual interest. Metallics and holographic thread and heavier weights like 12 wt cottons, rayons or combos. These are normally used in art quilts, show quilts or wall hangings. Here is a link to a wall hanging I did that has many of the threads I have mentioned above used in it. https://www.quiltingboard.com/pictures-f5/quan-yin-show-quilt-t297585.html
Superior offers "try me" mini cones for a lot of their threads (they may offer it for all of their threads). The price is discounted but you can't select what color. This is a good way to try several different types of threads. Metro is also very economical. https://metroemb.com/store/index.php?route=common/home When I attend large national quilt shows I usually pick up threads at these. Often at the shows they offer specials like buy so many get one free.
I would be remiss not to mention that with thread education comes needle education. You need to use the correct size needle for the type of thread you are using.