A jacket that is part of a suit and a jacket that works with other clothing are two different items, though occasionally there is some interchangeability. There are several reasons why jackets often don’t work when worn with different clothing.
Jackets tend to have a looser weave. Many suits tend to have a denser weave that keep it shape. This is sometimes interpreted to mean the jacket material will be soft, but this is not always the case; silk, wool, cashmere and Shetland all make good jackets, but vary greatly in softness. They can all still be quite pliable.
Fibres for these jackets tend to be longer in type; cashmere and other materials can be used if the longer fibres are hand chosen. The finishing of a suit, the process by where the surface is made shiny, fluffy or otherwise, notable alters the look of a garment. Suits are almost always shiny or fine texture rough, but never fluffy. Jackets tend to be less shiny and have a fluffy, or at least soft look.
Suits rarely have patterns, or at least not prominent ones. The jacket and trousers of a suit must match, and a pattern that covered both of these would be too dominant. On the other hand, a jacket tend to be a contrast to the trousers, so a pattern on a jacket can go well with plain leggings.
Some blazers are a little closer to a suit jacket, having a tight weave and shiny finish. They are cut almost like a slightly looser version of a suit. Other blazers have the un-shiny, soft finish of a jacket though they still look almost formal.
A good quality suit or jacket can be altered to fit well or just altered for stylistic preference. But the basic material of the clothing is obviously set. Consider the fundamentals of the formal clothing you buy. The details can be altered to your needs, but the basic material and shape are set. It help to have a knowledge of what alterations can and cannot achieve.