While the "WEB-DL" terminology is typically used by file-sharing communities to describe content extracted directly from streaming services, the official technical specifications for on Prime Video include: Resolution: 4K UHD (Ultra High Definition). High Dynamic Range: HDR10 and Dolby Vision. Audio: Dolby Digital 5.1 (DD5.1) and Dolby Atmos .
This refers to (usually HEVC/H.265).