What is the grammatical explanation of the phrase "all that" as in the sentence "I didn't understand all that much"

What is the grammatical explanation of the phrase “all that” as in the sentence “I didn’t understand all that much”?

1. I didn’t understand all that much.

I came across the sentence above, and now wonder if someone could explain the grammar about the phrase “all that”.

For example:

2. All that glitters is not gold.

In this case, ‘all’ seems a pronoun; ‘that’, a relative pronoun. But ‘that’ in the sentence (1) does not introduce a relative clause. Therefore, the usage may be different.

I do not have any other clues to find out the grammatical explanation.

After discussing with John, I think it is a good idea to add another sentence for further understanding.

3. I didn't understand all that.

This sentence is very similar to (1), but it could be structurally different from (1).

I hope this comparison help us understand these usage more precisely.

Solution 1:

(All) that much is a quantifier, but more importantly it's an idiomatic Negative Polarity Item.

That is, it can only occur felicitously within the scope of a negative trigger.
Like didn’t. Notice that you can certainly say

• I didn't understand all that much of what she said.

but you can't say

• *I understood all that much of what she said.

because there is no negative trigger present.

In the appropriate negative context, it means, basically, not much at all

• I doubt he enjoyed all that much of the play.
• It isn't as if she weighed all that much.

And it can also be used with other quantifiers and qualifiers besides much:

• She isn't all that gorgeous.
• If it's all that important, why don't we hear more about it?

Solution 2:

1. All is an adjective; to be more precise, a determiner. That is a demonstrative pronoun that functions as a noun; it is modified by all. The construction is similar to all that stuff, except that that functions as an adjective in that sentence, modifying stuff. Here we don't have stuff, so that takes the place of a noun.

2. All is an adjective/determiner that functions as a noun; many adjectives can function like nouns, such as in the weak perished and red is nice. That is a relative pronoun referring back to all, its antecedent. It introduces the relative clause that glitters. The main clause is all [that glitters] is not gold: the relative clause is restricting/defining, because it restricts what all refers to. Without it, all would refer to more things than with the restrictive relative clause.