Cracked Minecraft probably exist for as long as the ordinary Minecraft does.
You can play both single- and multiplayer (only limited to cracked server). Some awesome features are only available to those who has bought the game, though.
Anyway, if you like this game, consider buying it :)
Can I install mods?
Yeah, sure!
There are popular Forge and Fabric versions in the launcher that can be installed automatically in just a few clicks. You can also install Forge and Fabric manually, just like with the official launcher.
Can I change my skin in Legacy Launcher?
Yeah, sure!
You and your friends should use a free Ely.by account. Check out Accounts menu in the launcher.