1. Songpyeon is the food I like best.
2. Songpyeon is a rice cake which I like most.
3. Songpyeon is what I love to eat.
4. This is songpyeon, which I like best.
5. This is the Opera House, which operas are played in.
6. This is the Opera House, which is in Sydney.
7. This is the Opera House, at which people get together to watch operas.
8. This is the Opera House, which people love to watch operas at.
(Would you check the sentences? Are they all grammatical?)
All are fine except 5 and 8.
My suggestions:
5. This is the Opera House where operas are performed.
8. This is the Opera House in which people love to watch operas.
Yes, all of the sentences you provided are grammatically correct. They all use relative clauses to add additional information about the noun they are modifying.
In sentences 1, 2, 3, and 4, the relative clauses provide information about "Songpyeon." The relative pronoun "which" or the pronoun "that" connects the relative clause to the main clause.
In sentences 5, 6, 7, and 8, the relative clauses provide information about "the Opera House." In these sentences, the relative pronoun "which" is used to connect the relative clause to the main clause, and additional prepositional phrases are included.