it's here; 10 USC §118.

It starts:
(a) Review Required.— The Secretary of Defense shall every four years, during a year following a year evenly divisible by four, conduct a comprehensive examination (to be known as a “quadrennial defense review”) of the national defense strategy, force structure, force modernization plans, infrastructure, budget plan, and other elements of the defense program and policies of the United States with a view toward determining and expressing the defense strategy of the United States and establishing a defense program for the next 20 years. Each such quadrennial defense review shall be conducted in consultation with the Chairman of the Joint Chiefs of Staff.
Note the time period to be forecast.

Here's Congresses big issue:
. . .
(3) to identify
(A) the budget plan that would be required to provide sufficient resources to execute successfully the full range of missions called for in that national defense strategy at a low-to-moderate level of risk, and
(B) any additional resources (beyond those programmed in the current future-years defense program) required to achieve such a level of risk; and
(4) to make recommendations that are not constrained to comply with the budget submitted to Congress by the President pursuant to section 1105 of title 31. (emphasis added / kw)
The first highlight is their notice that they aren't going to fund everything everyone wants and 'they' are willing to accept some risk (nice of them...). The second highlighted item is their notice they're going to fund what they want...

The QDR sprang from the post Cold War Dividend mentality in the mind of then SecDef Les Aspin (not as bad as Louis Johnson but bad enough to make Rumsfeld look like a gentle genius) and his Bottom Up Review (BUR) looking for that peace dividend. The BUR was the first appearance of the idea that the US should be able to fight two “nearly simultaneous major theater wars,” a riff on the the two-big-war standard of the Cold War. The bottom line of the idea was to justify major defense spending and force structure cuts. The BUR was seized upon by Congress as a way to get more spending clout than they already had so the QDR was born. It is essentially a waste of money though some good comes of it. Not much, some. I'll also note that as I said before; they never funded DoD to fight two wars (the mid-Reagan years came close).

It's all DC smoke and mirrors...