Luke 3:23: “Now Jesus Himself began His ministry at about thirty years of age, being (as was supposed) ...” (NKJV)
Berean Study Bible
Jesus Himself was about thirty years old when He began His ministry. He was regarded as the son of Joseph, the son of Heli,
In the original Greek, it is just a word. It carries a sense of being generally accepted fact that Jesus was the son of Joseph, in the indicative mood.
Verb - Imperfect Indicative Middle or Passive - 3rd Person Singular
Strong's Greek 3543: From nomos; properly, to do by law, i.e. To accustom; by extension, to deem or regard.
Definition: to practice, consider
Usage: I practice, hold by custom; I deem, think, consider, suppose.
Thayer's Greek Lexicon
to hold by custom or usage, own as a custom or usage; to follow custom or usage; passive νομίζεται it is the custom, it is the received usage: οὗ ἐνομίζετο προσευχή εἶναι, where according to custom was a place of prayer
to deem, think, suppose: followed by an infinitive, Acts 8:20; 1 Corinthians 7:36; followed by an accusative with an infinitive, Luke 2:44; Acts 7:25; Acts 14:19; Acts 16:(18 (see 1 above)), Acts 16:27; Acts 17:29; 1 Corinthians 7:26; 1 Timothy 6:5; followed by ὅτι, Matthew 5:17; Matthew 10:34 (Winer's Grammar, § 56, 1 b.); Matthew 20:10; Acts 21:29; ὡς ἐνομίζετο, as was accustomed to be supposed, Luke 3:23. (Synonym: see ἡγέομαι, at the end.)
Luke used this Greek word to hint that Joseph was not the biological father but a legal one.
Barnes' Notes on the Bible
As was supposed - As was commonly thought, or perhaps being legally reckoned as his son.
John 6:42 confirms this common knowledge:
They said, "Is this not Jesus, the son of Joseph, whose father and mother we know? How can he now say, 'I came down from heaven'?"