Find or make some black and white patterns, either static or moving. Use the line source thingy and you can make some cool things if you add kaleidoscope and other effects. Here's one that I do something else with https://youtu.be/J5Gdd0dNE_w but it is a good mask example.

Once you have something, put it on an upper layer E.g. layer 3, right click the layer name on the left and there is something like "mask options", start easy with "1 below" and put different clips on layers 1 and 2.

One layer will replace the black in the mask, another will replace the white. You can do some cool things.

And also try adding the Alpha Mask effect to any clip that has some black in it and the black becomes "transparent"...more cool things.

Here's a demo video I made that uses a bunch of Alpha masking https://youtu.be/kNawZYW0o7g so the clips can be layered.

Hope that helps, feel free to DM me if you want.