대다 is putting one thing against another, and 갖다 대다 — 'bring it over and put it against' — is the compound doing the work here: 골전도 이어폰이라고 하는데 머릿쪽에 이렇게 갖다 대서 'they're bone-conduction earphones, so you hold them against your head', 발을 전화기처럼 이렇게 귀에다 갖다대고 'holding a foot to the ear like a phone'. 입만 갖다 대다 is 'just put your lips to it' — barely taste it.