Renovating old house in VIC - Certificates for electrical & plumbing

Hi guys

I'm from Sydney so please forgive my silly questions about how things work in Vic.

I understand contractors can issue "certificates" for the following work:
- electrical
- plumbing
- gas (not touching gas so I won't comment on this in my post)

Can someone please tell me what these certificates are, what information do they contain, what is their purpose? I suspect it's something to do with confirming works were done by licensed people in case people sue me but please enlighten me!

Do most landlords in Vic ensure they have all such certificates when renovating an IP?

ELECTRICAL CERTIFICATE

I understand for electrical works, am I correct in believing you can get a prescription certificate or a non-prescription certificate?

Is a prescription certificate when the government inspector signs off on it?

Is non-prescription certificate a certificate issued by licensed electrician setting out the works done to the property but without the government inspector signing off?

To get either a prescription certificate, does the old switchboard have to be completely replaced?

To get a non prescription certificate, is it enough for me to just have the old switchboard "upgraded" [if that's a possibility]?

Is there a big price difference between getting a prescription or non-prescription cert?

PLUMBING CERTIFICATE

I assume this is to confirm all plumbing & waterproofing are done properly?

Is there prescription vs non prescription certs for plumbing?

How do the contractors get the cert? Do they have to show they've used licensed people?

Does everyone get them? I heard some people don't bother getting them for PPOR when using referral contractors who have done other people's properties without issue.

Thank you