nearest neighbor upscaling in Davinci resolve (read desc)
*********
If you are upscaling and want the footage to have a more realistic softness to it, it's probably best to use the zoom x2 with bilinear, or the scale node set to "linear", as it will keep the natural blur that footage usually has. In my case i wanted a sharper look.
**********
After a while of testing, i've concluded that using the scale node in the fusion tab is the most accurate way of upscaling footage, for my goal of making not losing the original 1080p quality that the footage has, even when the new 4k footage is downscaled