These are from the page for 'as' as conjunction on Oxford Dictionary. I don't understand why they are inverted like 'as is her mother' or 'as do I'. Why not 'as her mother is' or 'as I do'?
I've searched the net a little and people are saying that 'as do I' is more formal than 'so do I', but is that really so? I've learned 'so do I' to mean 'me too' long time ago in the official text book written by the government in my country, but I don't think I ever learned 'as do I'. I've just studied that 'as' can be a relative pronoun but replacing 'which' to 'as' doesn't make sense for these two sentences, nor giving 'it' after 'as', so I know they are not relative pronoun, but what are they?
4 used to make a comment or to add information about what you have just said
• As you know, Julia is leaving soon.
• She's very tall, as is her mother.
As is a conjunction and an adverb and is used before a clause, another adverb, or a phrase beginning with a preposition: She enjoys all kinds of music, as do I. As always, he said little.