Yeah I mean technically we can't say anything is actually zero but the lower bound is pretty low.

It depends! There is often a difference between bulk and thin film superconductors, and it will depend on loads of details relating to fabrication and the frequency of signals present.

For example, a resonator made of thin film Nb has zero DC resistance but will have dielectrics and other contaminants that give it some loss tangent when in the presence of microwave fields.

We usually talk about "Quality factor" which is sort of a way of saying how many times can a microwave photon bounce back and forth in the circuit before it leaves due to some loss mechanism.

Qs of tens of millions are achievable.

It totally depends on the superconducting metal and the impurity.

Superconductors have a tendency of "proximatizing" other materials, making them superconducting by being near to them.

For example, a 1nm thin film of a normal metal on top of a 100nm thick film of superconducting Niobium will likely superconduct.

Similarly, some superconducting metals when deposited with non-metals actually can become better superconductors. For example, Aluminum that has a little oxygen in it (granular aluminum, or dirty aluminum) actually has a higher superconducting critical temperature than clean aluminum.

That said, if you get too much copper or gold in your superconducting film, it just won't superconduct at all.

The interactions are all really complex and involve the coupling between the lattice of the superconducting metal and the charge carriers.

Look up "BCS theory" for more info!

Source: PhD working in superconducting qubits.