Keeping the original firmware but installing a patch (like HelloOX2 ) that gives you "Read/Write" access to protected system folders (C:\sys, C:\resource).
Unlike modern Android ROMs, S60v3 "custom ROMs" are often modified versions of official Nokia firmwares rather than entirely new operating systems. s60v3 rom
Today, most S60v3 ROM development has moved to archival sites like All About Symbian or dedicated Discord communities, as the original forums (like DailyMobile or Symbian-Freak) have largely disappeared. Keeping the original firmware but installing a patch