a lot of times, if you reencode the file as uncompressed ie. png quicktime non-gop video, these problems go away. Only the latest Ae supports compressed video imports.
collapse transformation/continuously rasterize switch(looks like a sun) should keep vectors crisp. Now if they’re not vectors, well then its up to a good bicubic resize plugin.
don’t forget, unlike prores’ 444 10 bit (trillions of color); animation and png in the quicktime container only support 8 bit (millions) color so if you do any color manipulation, there’s a small error pixel shift.
well i would google adobe’s white paper on color management. and I could rattle on about support across browsers, viewers, but for the sake of simplicity, interpreted support is sRGB as the default color management for most browsers.
here’s a fun little read which seems to be not too far off what you’re probably asking about
i’d make a luma matte(contrasted with effect-levels) from the existing footage. this will make the ground pop through just like in real life. add, screen, with various opacities etc. if you need tracking, mocha or syntheyes.