You can go to Settings > Display and Check if there is a cast option. If there is an option, select cast option, and select a nearby device to cast upon.
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Galaxy S8 vs OnePlus 5
Both the phones have their pros and cons, I think you should go for OnePlus 5. OnePlus 5 has the following advantages over Galaxy S8 -
But it's not that Galaxy S8 does not have any advantages over OnePlus 5:-
There are many things which are common between them one of them is their processor's performance.
Verdict:- OnePlus 5 has many advantages over Galaxy S8 such as more RAM, more storage, Dual camera and the best is you are getting all these features and specs at a very good and relatively lesser price than Galaxy S8.
You can check the complete comparison on 91mobiles link:-OnePlus 5 vs Galaxy S8