Most repacked games cannot access official Steam servers for multiplayer unless they use specific "online fixes" that simulate a legitimate connection.
"Steam Repacks" are compressed versions of PC games originally distributed on the Steam platform. They are primarily created by third-party individuals or groups to reduce the initial download size, making games more accessible for users with slow internet or limited storage. What is a Steam Repack? Steam-Repacks
PSA : How to install a repack on your SteamDeck running SteamOS Most repacked games cannot access official Steam servers
Repackers use tools to shrink game data. For instance, a 100GB game might be compressed into a 30GB to 50GB installer. What is a Steam Repack
Because the files are so tightly packed, the installation process requires significant CPU power and time to decompress and extract the files to their original state.
Repacks are technically a form of software piracy if they distribute copyrighted game files without permission. While some users "buy the game but download the repack" to save bandwidth, the distribution and use of these files often violate EULAs and copyright laws.