it indicates passive voice, it can be omitted sometimes for ex: 欧盟由XX国家所组成 = XX国家组成欧盟 EU is made up of several countries 这辆车由我所拥有 = 我拥有这辆车 The car is owned by me

For example, 白色是我所喜欢的颜色。

所is more like "from" The European nation is made from ·····

No, it should be a structural particle. 结构助词放在词、词组的前面或后面，用以表示句子的结构关系。常见的结构助词有前缀的“所”，以及后附的“的、地、得、似的”等等。