grammar - Explanation of "did was [verb]" structure

All I did was hand someone a bag.

There are three verbs that come together. The tense of the verb hand confuses me. It doesn't fall into any grammar structure I know so far. It would make more sense to me if it were "All I did was handing..." (because handing is a noun form of the verb) or at least "All I did was handed..." (because it might be equal to "All I did was I handed...").

Answer

First, let's look at a similar sentence:

1a. [ All I wanted ]　was 〔an ice cream cone〕.
1b. [ The only thing I wanted ]　was 〔an ice cream cone〕.

In example 1a, all I wanted is a noun phrase.
It means the only thing I wanted, so example 1b means the same thing.

Your example is similar:

​2. [ All I did ]　was 〔hand someone a bag〕.

Here, the bare infinitival clause hand someone a bag is used as a complement of specifying be.
This is one of the few functions of bare infinitival clauses—they appear mainly as complements of a few specific verbs, and one of those verbs is be.

If you'd like, you can call it a nominal bare infinitive clause, because it appears where a noun phrase would normally be expected. But in any case, this construction is grammatical and totally normal.