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EUREKA!!!I figured it out!
By myself and with my own mind (almost – thanks to HEROESWORLD).
Those who knew everything (Xaker, for example :smile31:) could have shared all the details.
But since everyone is silent, I will tell you.
Go into the data folder, then into data.pak/Maps. Look for something there. Next, I'll give an example using Contested Island (Maps/Multiplayer/A1M3/A1m3). I simply extracted it from there. Created a1m3_1.zip/Maps/Multiplayer/A1M3/A1M3/{all files here}. Renamed a1m3_1.zip to a1m3_1.h5m. It didn't work. Then I "cut out" one of the folders with the name A1M3, resulting in a1m3_1.zip/Maps/Multiplayer/A1M3/}{all files here}. Again, I changed the extension to h5m, put it in maps and voilà! Now I can edit Contested Island however I want. However, it will naturally be a separate map. And there are also some problems with the texts. More precisely, there are no texts (there is no problem:)). They need to be rewritten (or found somewhere in texts.pak and copied from there).
Read about it here: How to create Mods.
If something is wrong, correct me (I checked this purely empirically).