Simple masking How To question

Be kind - just loaded Resolve on my computer this week - minimal editing experience. I would like to create a mask whose attributes (shape, size, position) remain fixed and independent from the video that shows, meaning that I want to be able to move and zoom the video but not the mask. Is this possible? Everything I try results in both the mask and the video changing - which is great for a lot of circumstances but not what I want in this particular case.

If the mask (Power Window) is done on the Timeline layer, it won't move if you do an Input Sizing change on the Color page. You can keyframe the timeline layer so that your effect doesn't affect anything other than that shot... but you have to make absolutely sure you turn it off elsewhere in the timeline.

You can also introduce a shape as an image file on a track on the Edit page, and I think if you change the Edit sizing of one clip it shouldn't affect any other clips. So there are multiple ways to do this.

Let me elaborate to what Mark said, since you are starting out:
First Method: On the color page, above the node window you see a menu item "Clip", change it to "Timeline", then add node Corrector and connect green input and output. Now if you make a power window in that node it affects the entire timeline.
Second Method: Make a psd, or png file in Photoshop and drop it on the edit page on a top track.
Or grab solid color from "Generators", drop it on the top track and use cropping features.
Or drop solid color on the top track, right click "make compound clip", then go to Color page and make a power window, "add Alpha output", and connect the blue mask output to Alpha output.
and perhaps there are other ways too....

a third way: drop any black and white image on top track on edit page, then change composite mode to "Multiply"