grammar - 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.

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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.