A .rar (Roshal ARchive) is a compressed folder. Due to the size of Switch games (Astral Chain is roughly 10GB), uploaders split the NSP into multiple parts. If you see astral chain nspupdate 101.rar, .r00, .r01, etc., these are split archives. You cannot install a .rar file directly. You must extract it first.
For Official Users:
For Homebrew/Unsigned Package Installation: astral chain nspupdate 101rar install
Warning: Installing games or updates through unofficial means can violate terms of service and may expose your device to risks. Verify: When finished, hover over Astral Chain on