I understood that i can download OSM file of an area I want to use offline.
So I download an area from OSM but I can’t find an app’ that I can upload this file/map and that can read it.
For example I tried to use it on ORUXMAP but don’t work
DO I do something wrong or in general this is can’t be done?
Generally speaking map apps need data in a format that they can understand, and that usually won’t be the raw OSM data that you can download from OSM directly. I’m not familiar enough with Oruxmaps to know what format that needs, unfortunately.
Specifically for Oruxmaps, would it be an option to use OpenAndroMaps (https://www.openandromaps.org)? This works very smoothly and avoids any need to create a map yourself, and is designed to work offline - I use this combination for hiking a lot. Whether it works for you depends on what you want to do, e.g. it won’t help if you need to instantly update changes in OpenStreetMap data.