In plain English
FAR 52.236-1, Performance of Work by the Contractor, is a contract clause prescribed at FAR 36.501(b), most recently dated Apr 1984. The complete official text is reproduced below, verbatim, from GSA's published FAR source files.
Its text contains no sentence requiring insertion into subcontracts; see the flowdown section below for what that does and does not mean.
Does it flow down to subcontracts?
No flowdown mandate found in the clause text
We scanned the full clause text and found no sentence directing the contractor to insert this clause into subcontracts. That is a statement about the text, not legal advice: a prime contractor may still flow terms down contractually, an agency supplement (DFARS, VAAR, …) may add requirements, and clauses listed below (if any) may order this clause into subcontracts from the outside.
Where it's prescribed
As prescribed in 36.501(b), insert the following clause: [Complete the clause by inserting the appropriate percentage consistent with the complexity and magnitude of the work and customary or necessary specialty subcontracting (see 36.501(a)).]
Prescribing reference: FAR 36.501(b), FAR 36.501(a).
The official text, verbatim
As prescribed in 36.501(b), insert the following clause: [Complete the clause by inserting the appropriate percentage consistent with the complexity and magnitude of the work and customary or necessary specialty subcontracting (see 36.501(a)).]
Performance of Work by the Contractor (Apr 1984)
The Contractor shall perform on the site, and with its own organization, work equivalent to at least _______ [insert the appropriate number in words followed by numerals in parentheses] percent of the total amount of work to be performed under the contract. This percentage may be reduced by a supplemental agreement to this contract if, during performing the work, the Contractor requests a reduction and the Contracting Officer determines that the reduction would be to the advantage of the Government.
(End of clause)