How to query relay chain height by a para chain height in fronted

Question

I find a similar question here, but it's for code running in a para chain.

My question is how can we get the relay chain block number by a para chain block? There are code like this in polkadot sdk.

let latest = RCBlockNumberProviderOf::<T::Coretime>::current_block_number();

How can we get the code work like this in fronted? I checked the coretime block 1363778.

1. Should we use the relay_parent_number in the parachainsystem(set_validation_data) extrinsic?
2. Is every block in a para chain has a parachainsystem(set_validation_data) extrinsic?