"Đội" is a marker used for clothing worn above the waist - hats, shirts etc. - while "mặc" is used for clothing items worn at or below the waist - belts, skirts, pants etc.
I would greatly appreciate being informed as to why "một" is not being utilized?
Yeah, this would be more of a general statement of something you usually wear. I wear hats, and pants, and shirts, and shoes, etc.
or similarly. "I like to wear hats" - "tôi thích đội mũ"
đội means "wear/to wear" If you want to inflect it to wearing / wore / will wear or even usually wear, you need to add more words which we will be learning soonish hopefully.