Click thelinkicon on the toolbar.
pop in or paste a URL, and clickApply.
you could also clickheadings and bookmarksto link within the current document.

The text is now a hyperlink.
Heres how to hyperlink in the Google Docs app:
Open a document in editing mode.
Tap the word you want to turn into a hyperlink.

key in or paste a URL, and tap thecheck mark.
Your text is now a hyperlink.
What Hyperlinks Work in Google Docs?

These different types of hyperlinks are all created using the methods described above.
First, place the cursor where you want the link to lead, and then go toInsert>Bookmark.
An icon will appear at that spot, and you’re able to selectCopy Linkto get the URL.

Click theSharebutton in the Doc you want to link to.
At the bottom of the window, selectCopy Link.
Then, fire up the second Google Doc and set up a hyperlink.

Note that everyone you want to see both items will need permission to read them.