S60v3 introduced "Platform Security." You could not install an app unless it had a valid Symbian Signed certificate. Custom ROMs patched the kernel to grant "All-TCB" (Trusted Computing Base) capabilities, allowing unsigned apps and hacked versions of Opera Mini or NGage games to run without permission pop-ups.
Professional-grade service software used to "flash" (write) the custom firmware files to the phone via USB. s60v3 rom
(Note: I can suggest related search terms to explore ROM downloads and guides.) S60v3 introduced "Platform Security
Step 5 — Prepare the phone and PC