Look at the website or forum where you found the link. Often, the "true" name of the file is listed in the post description or a separate .nfo file.
Tools like CheckFileType or the file command in Linux can sometimes identify the headers if the extension has been changed. RRRDDD.part12.rar
The .part12.rar extension indicates this is the 12th volume of a larger, multi-part RAR archive. To extract the content, you generally need all parts (part01, part02, etc.) in the same folder. Look at the website or forum where you found the link
Be extremely careful when opening multi-part RAR files with obscure names from the internet, as they are frequently used to distribute malware or ransomware . you generally need all parts (part01