It's unclear if you're asking about sewn slings or single lengths of webbing--so I'll answer each separately below.
The most important thing when using webbing (sewn loop slings or single lengths) is that any knots should leave the webbing laying flat. There should be as few twists as possible in your system, and none in the knot itself, as these decrease the effective tensile strength of the webbing and increase the likelihood of knot slippage.
Also, worth repeating what you say in the question--don't tie together dissimilar webbing, as it often won't hold a knot well.
In my (somewhat limited) experience, sewn slings are more popular because of the ease of clipping to a carabiner (or girth-hitching to a harness, more on this later).
End to end, single lengths of webbing
This website suggests using a water knot--essentially an overhand knot in two strips at once. It's incredibly important to a) leave long ends and b) tighten by hand before loading, because it can slip on initial loading.
I've used and seen this used for anchors or other permanent ties. It's nearly impossible to untie once used.
Loop in end, single length
The same website suggests an overhand on a bight. This is similar to the previous knot in that it is based on a simple overhand knot. Again, very challenging to untie.
"End to end", sewn sling"
Of course, sling loops don't actually have ends! And in fact, tying them together is not really a great idea. You can do so with a strop bend, explained here. While it is pretty much impossible for this knot to slip, it can vastly reduce the strength of the slings themselves.
This article from Black Diamond has some interesting tests about connecting slings together. Doing so with certain knots may reduce strength by 50%, which is a scary factor when your life is literally hanging on it.
Much simpler in this case is to use a locking carabiner. Lots of sport climbers do this, and while that doesn't mean it's safe, in my experience it's an accepted practice.
Loop in end, sling
Slings are loops! For your ascender, you can probably just clip in a long sling loop and call it a day.
If you need to loop a sling around a fixed thing, such as a metal pole, you can use a girth hitch.
