Strange uses of ‘do’

A: I need someone to help me and you'll do.
B: Thanks.
C: (Later, to A) Did B do?

I don’t think that would be said. ‘Was B good enough? Did B do a good job?’

‘do’ is the doing word, if any word is a doing word.

But here B is not doing anything. A is saying B is good enough, in their eyes.

Someone suggested,

'It will have to do'

Is that the same as,

'We had to do with black and white, 14-inch monitors'?

And, You can’t say, ‘I’ll do’ to mean ‘I’m good enough’, I don’t think.

‘I do’ is the grammatical use of do as an auxiliary.

And in this case, it’s intransitive. Usually it’s not. Not ‘Do!’ but ‘Do it!’

Is the ‘it’ll do’ also the auxiliary use?

A: What will this car do?
B: It'll do 200 kilometers per hour/40 kilometers per liter. Will that do?
A: Yes. I'll take it.

That’s more like the regular meaning of ‘do’.

Then there’s the idiom:

A: Can you mail this for me?
B: Will do.

Meaning, ‘I’ll do it’