The "Huan" in "There is nothing but nothing" is usually understood as "returning" or "recovering to". The whole sentence comes from Taoist thought and expresses a philosophical concept. That is to achieve a natural state through inaction, in which things seem to exist but transcend the conventional state of existence, and finally return to a state of nothingness.
The word "huan" in "The classics are still familiar with the things, and the reading of people is like reading the river" can be understood as the meaning of "still" or "still". This sentence comes from the poems of Du Fu, a poet of the Tang Dynasty. It means that by experiencing various things, a person will gradually become familiar with the world, and after reading countless people, his understanding of people will be as deep and broad as reading a river.
The specific meanings of the two "huan" in the context are slightly different, but they both involve the impact of experience and time on knowledge and wisdom.