How did adam and eve get grandchildren?

Answer 1

Two grandchildren specifically were mentioned for Adam and Eve in the book of Genesis. One was Enoch, as Cain's son and Adam's and Eve's grandson [Genesis 4:17]. The other was Enosh, as Seth's son and the couple's grandson [4:26].

Adam and Eve obviously had significantly more grandchildren than are recorded. The ones we do know about are:

Enoch, son of Cain (Genesis 4:17)

Enos, son of Seth (Genesis 4:26)

According to Genesis 5:7 Seth had sons and daughters (after Enos) and so Adam had an absolute minimum of 5 from that line.

Adam himself had sons and daughters (most likely apart from those referred to) and so there are also grandchildren from those lines, although of course at that early time people were marrying within their own family. So sons of both Cain and Seth were 'doubly' related also through the maternal line.
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Answer 2

There is only one person that is written in The Bible and that is Enosh

Answer 3

Enoch (from Cain), Enosh (from Seth), and many, many others not mentioned were the grandchildren of Adam and Eve.

Answer 4

Their grand children came from their children.

Answer 5

According to the bible adam and Eve had other sons and daughters, so Cain married his sister, and the family tree continued.