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

• 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}