[cperciva]

What do the authors do if a big corp asks for support regarding something that only happens at scale?

I don't see the problem here. Presumably a big corp could afford to pay enough for support that a developer would be inclined to help even if said big corp was the only user encountering a particular problem.

[watwut]

It is more likely that big corp will have their own team fix the problem, unless the original code is too big or convoluted.

[pjmlp]

The majority of big corp outside IT world just use consulting.

If they happen to have an internal team, it is mostly to manage contractors and architecture decisions for what they want to get delivered to them.

[watwut]

I would count consultant as "own team" in this scenario. Someone company controls at that time, knows company code/cases and is available more or less instantly. A lot depends on how much special knowledge is needed to understand open source project codebase etc. Ordering contractors you already know around is easier then ordering someone on other side of the world who has different employer and priorities.