Depending on what is inside the .rar , you will need to move the files to one of two places:
It often contains .fmf files that move players to their modern clubs or unlock "greyed out" national teams like Germany or Japan. 🛠️ How to use the contents sc15370-FM21v2140.rar
Only download files like this from reputable community hubs like FMScout or SortitoutSI . Depending on what is inside the