A relay is used for turning 12 V on/off for high current users such as headlights, because a contact on the handlebar maybe can't take that high current or it has an internal resistance, so the voltage drops.
For small current use, such as phone chargers, you don't need a relay. USB charges with 0.5A, a few with 2A, so a 0.75mm square wire is sufficient. A fuse is alway a good idea, to safeguard the wiring from fire, if its grounded somewhere.
May you enjoy peace and good health !