How do 'within' and 'which' form a relative pronoun clause?

Routines offer a structure within which to prepare for performance.

I'm having trouble untangling the relative pronoun clause into a sentence of its own. At first glance, the two sentences combined seem to be simple. However, when attempted, there seems to be no subject for the relative pronoun clause.

Routines offer a structure.

Within the structure to prepare for performance. (lacks subject)

This has me questioning whether or not the original sentence is grammatically correct, which seems intuitively implausible. Any tips?

• Routines offer a structure within which to prepare for performance.

This is a case of postmodification by infinitive clause (CoGEL p. 1265 § 17.30).

Unlike -ing and -ed clauses, infinitive clauses as postmodifiers in noun phrases allow correspondences with relative clauses where the relative pronoun can be not only subject, but also object or adverbial and, to a limited extent, complement:

S: The man to help you is Mr Johnson. ['who can help you'] [1]
O: The man (for you) to see is Mr Johnson. ['Who(m) you should see'] [2]
[…]
Unlike [1], [2] can have an optional subject of the infinitive clause introduced by for. Without such a subject,the infinitive clause in [2] could be understood, according to context, as ' (The man) that you/he/everyone, etc. should see'.

There is not actually a relative clause (for which a subject would be necessary), but there exists a correspondence with a relative clause.

Corresponding relative clause

• Routines offer a structure within which everyone/they/… can¹ prepare(s) for performance. ("Which" is an adverbial.)

¹ _{Modal added after remark by user John Lawler}