you can get something like this
Bluetooth Music Receiver | Belkin TH
the onkyo would likely have a aux/cd input photo connector on the back which you then connect the belkin cable to and bobs your uncle
of cause there are other brands then belkin for products like this
bluetooth music receiver - Google-
if you wish to do it by wire then this one
Buy USB-C to 3.5 mm Headphone Jack Adapter - Apple
and then connect it to something like this
minijack to rca cable - Google-
with the proper gender connectors in each end, to match your receiver and the adaptor