Numbering 123 is a list style, i.e. it deals only with the appearance of the number/bullet and its position over levels.
List 1 is a paragraph style, i.e. it describes the look of item text (essentially font properties and spacing above/below).
A list (item) is implemented by a combination of a paragraph style and a list style. Conceptually, the list style tells to which “logical” list the item belongs.
If you analysed carefully my sample file, you have noticed that the bullet list is styled List 1.
Buikt-in styles List n (and “cousins” List n Start and others) are intended for bullet lists while Numbering n (and others) for numbered lists. However Writer and its developers have no idea about which type of bullet/numbering you prefer. Consequently, no list style have been associated with theses paragraph styles. To turns them int styles for lists, you must assign a list style to them, either manually (a kind of direct formatting), or by modifying the paragraph style to associate it with a list style.
Not exactly. Your question asked for an automatic solution. What I described is the nearest approximation to “automatic”. You can proceed with a more manual procedure. In this manual procedure, you have a single paragraph style (with its companion list style). If you want to change one list item, apply a character style over all item text. This is perhaps easier for a beginner (in list formatting domain).
Either you follow my suggestion: several paragraph styles associated with a single list style. You decide which paragraph style to apply to the item. Applying another paragraph styles will not change numbering, only the look of the item, because they are all associated with the same list style.
Or you work with a single paragraph style (+ its list style) and you apply a specific character style(e.g. Strong Emphasis for bold or Emphasis for italic) or no character style for raw paragraph style look (in principle regular). IMHO, it is more complicated and could result in more fuss if you want to enhance or or words even differently. Modifying the base highlight becomes then more difficult.