Learn English – “Do” question meaning

Is this supposed to be fixed by us?

This is the grammatically correct version since the affirmative statement is:

This version is supposed to be fixed by us.

However, I don't know why you would use the passive voice here. The best, most direct version is

Are we supposed to fix this version?

Where did you find it?

Yes, in such questions you should use the verb do. Note that after do you should use the bare form of the verb: find, not found. The verb do carries the Past Tense, being transformed into did, so there's no need for find to carry the Past Tense too. The two verbs work together, after all.

If you put this question into the Present Perfect, for example, you would need to use have:

Where have you found it?

Both do and have are "auxiliary verbs".

If you have a statement

I have found it! (Present Perfect)

..and you want to transform it into a question, then you have your auxiliary verb ready: it's have:

Where have you found it?

But if you have a statement

I found it! (Past Simple)

There's no auxiliary verb in it. But to pose a question you need an auxiliary verb! What to do? You have to use do. This is called "do-support":

Where did you find it?

So it all depends on whether there's an auxiliary verb of some kind in the statement you want to transform into a question. If there's no such verb, use do.

But there's an exception: in some questions, there's no need for an auxiliary verb:

1. Who found it?
2. How many people found it?

It's because the pronoun who serves as the subject of sentence 1, and the question words "how many" in sentence 2 refer to the subject of that sentence, which is people.

P.S. The clause

Where you found it.

Will not work as a question. It might be included into some sentence, like this:

Remember that golden coin? I visited the place where you found it, and found another.