There are a few things that are not efficient in your workflow and your approach.
Lets talk first about Illustrator files brought into After Effects. First absolutely the only reason you would ever convert an illustrator layer to a shape would be to use animator tools found under the Add menu at the top of the shape layer properties in the timeline or to animate a bezier path on that layer. There is no other advantage to converting a vector layer to a shape. NONE.
Second, if you want to mask an adjustment layer then you can either draw the mask directly on the adjustment layer or put any layer with transparency, including an illustrator layer, photoshop layer, any masked layer or shape layer directly above the adjustment layer and use that layer with transparency as a Alpha or Inverted Alpha track matte for the Adjustment layer.
You can use the luminance values of any layer that doesn't have transparency as a luma or inverted luma track matte. White areas deliver transparency with standard luma. IOW, if you have a radial gradient with white in the middle and black on the edge and you use that as a luma track matte then the white will be transparent on layer below.
Standard track mattes only effect the layer immediately below the source.
Adjustment layers effect every layer directly below the adjustment layer. Mattes and Masks can be used to control the effects of an adjustment layer.
You can also use masks on any layer to control the any effect added to that layer if you have a recent version of AE. You'll find the option under Compositing options.
If you have any layer with transparency or strong luminance values you can use Auto Trace to generate a matte for that layer based on transparency or luminance. Any mask or bezier shape path can be copied and pasted to another layer.
Let's take your last question now. You should be able to answer it. To use any shape you have created as a mask you use that layer as a track matte.
Let's modify that approach a bit and assume that you have created a shape layer using the pen tool or converted an illustrator layer to a shape. You have a path that you can find by selecting the shape layer and pressing the U key twice. Find the path you want to copy, spin down the path options to reveal the animation controls and set a single keyframe and then copy (Ctrl/Cmnd + c). Even shapes created with the parametric tools like star, ellipse, rectangle can be converted to a path by right clicking on the shape and selecting convert to paths. When you have copied the path go to the layer where you want to paste the path, select the pen tool to start drawing a mask, click anywhere on the layer and paste (ctrl/cmnd + v) and you have created the mask. This works for shapes and for masks on any layer.
Here's one caveat about copying and pasting masks. The size and position of a path is calculated from the center of the layer and has no relationship to the scale, rotation or position of the layer. If you have drawn a mask on a layer that is scaled to 50% and moved to the top right corner of the comp and rotated 45º and then paste that path to any other layer you will NOT get a path in the same position and at the same size as the sampled layer in the comp. You end up with a path that would match the sampled layer if the all of the transform properties were reset and the layer was at comp center and the rotation and scale were normal. You have to be careful about that when copying and pasting bezier paths.
I kind of missed one question. If you nest or pre-compose a layer and there is an adjustment layer in the main comp it will effect all of the layers below it. If you pre-compose a layer and an adjustment layer above it then only the layers in the nested comp will be influenced by the adjustment layer. I kind of answered that before but wanted to make sure.
BE very careful when you look at YouTube tutorials on workflow because a growing number of them are teaching poor techniques and pointing you down dead end alleys. Make sure you vet your training.
If you have further questions the best place to find good community resources is to type a suitable word in the Search Help field at the top right corner of AE. For example, if you had typed adjustment layers or masks you would find a bunch of pretty good advice on how to use those features.
Thanks Rick, this is very helpful.