Would you say anything about this? VC At left is from water heater. Or would you advise installing a Y manifold. I guess what I should have done, was run the hot water until the water heater went on, turn up the thermostat on the forced air furnace, and then hold a smoking match or mirror to the water heater to check for backdrafting. Is that what you would have done? There was no evidence of any backdrafting.

What’s a VC and what’s a Y manifold? If that is a gas appliance vent duct tape does not belong. Terms for round pipe I understand can be found here
Transitions & Connectors - Ducting & Venting - The Home Depot

Thank you Erik for responding.
VC= Vent connector
Y= Wye

Am I using the wrong terms?
Yes, gas, and I did notice that duct tape.

So can anyone tell me what I should say about this? Thanks

A better picture would help, if the pipe on the left with the elbow is the chimney and there are no dips the connection (a Tee sometimes called a bullhead) is OK, but there is all that duct tape which does not belong, so you would be safe to say it needs a qualified person to fix it.

Here’s another picture, thanks.

Are they using the duct tape to encapsulate asbestos?

Duct tape does not belong, Tee connection is OK. Single wall pipe gas appliance vent connectors need at least 6 inches clearance from combustibles, looks as if the furnace connector does not have that. Get an HVAC or gas appliance tech to fix.