If you have standby redologs configured with the LGWR as your transport process, all the changes are already there (on the standby). If you don't have SRLs, and use ARCH process - you never receive changes in the primary's online redo logs (until the logswitch). It all boils down to your protection modes - maximum availability or maximum performance.

If I shutdown primary I assume new arc logs will be produced if necessary so that after the shutdown I have all the logs necessary for complete recovery.
AFAIk, no such thing happens.