'Do' and 'does' are the two forms in the existing indefinite tense with the verb 'to complete.' Which a person you can use will mainly depend on the sentence's matter. Their utilization is sort of similar, apart from 1 occasion where You need to use 'do' and not 'does.' https://financefeeds.com/the-biggest-threat-to-bitcoin-is-cardano-says-cardano-founder-experts-believe-it-might-be-this-copyright-instead/