more people come to America,mainly Europeas：:更多的人来到美国,这些人主要是欧洲人.mainly Europeans可以转换为定语从句：who are mainly Europeans.mainly的确是副词,而且是范围副词,我觉得它修饰are（虽然这么说有些不当,但我给不出更好的理由）.原句省略了who are.
The draft annimals in the procession are mainly camels
队伍中驼重物的动物主要是骆驼.