OPPO A57 archive of 12.3 MB in rar format; the name gives only the phone model. The larger size suggests it may bundle several documents, for example schematics together with layout or service pages, though this has to be checked after extraction. Schematics support tracing rails and signal paths, and layout files opened in a boardview viewer show component and test point locations. Make sure the documentation matches the board revision in your phone, which the name does not state.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @SMART_PHONE_SCHEMATICS. Enginerys does not host this file.
How to open a .rar file
Compressed archive — the usual wrapper for a schematic + boardview pair or a BIOS + EC set.