There's multiple ways of making a background that I've seen people use. Especially if it's just for reference by you, the author to flavor the characters beyond bland.
There's filling out a ttrpg sheet route like a D&D character sheet.
There's the vruber debut route. Which is origin, likes, dislikes, skills and talents, and goals. All just written down.
Frank Herbert tended to just have a pile of notes.
Heck you could even use a resume and dating app templates.
The format doesn't matter so long as you have enough info on the character that it can help guide you in writing that character's interactions with people and the world. The reader doesn't need to see the background, as it's a guidepost for you. Things like "is a light weight with booze" doesn't need to be said, as we'll find out if they get drunk off one or two drinks. Autistic, ADHD, sociopath, psychopath, extrovert, easily jealous, needs external validation.etc.
Let's take Luffy from One Piece for example:
Origin - Son the revolution army leader Monkey D Dragon. Likes pirates because of Shanks, who lost his arm saving Luffy from a shark.
Likes - meat, feasts, his friends, fun people, and nice people.
Dislikes - people who endanger his friends, people who talk shit, evil people.
Traits- rubber man from eating the gomu gomu devil fruit. Simple minded. Free Spirit. Loyal to the point of risking death for friends. Is a bleeding heart lead by emotion. Has a stamina based on how well fed he is. Rather asexual to the point even devil fruit charm doesn't affect him.
Weaknesses - sea water because of the devil fruit. Math, history, manners, general knowledge. Hard headed to to point of risking his life needlessly.
Goals - obtain the One Piece. Help friends and people he likes. Eat good food. Have fun and adventure.