There are over 100 million family trees on Ancestry, most of which are public. Family trees can contain a wealth of information, but they're only as accurate as the tree creators made them. Trees created by Ancestry members are not checked for accuracy. 

From a browser:
 

Search Public Member Trees

(or from any page on Ancestry, select Search > Public Member Trees)
 

1. Enter information about someone you want to find and click Search.
2. From the list of search results, select a name to learn more. To see all trees containing that person, select View all. (View all appears only if the person is in more than one tree. After selecting View all, select a name to see a particular version of that person.)
3. To see a whole tree or save someone to your tree, select Tools in the top-right corner and choose View in Tree or Save to Tree.
4. To contact the tree owner, select their username at the top of the tree or profile page > select Message.
5. To return to your search results, select the back arrow in your browser until you see the list of family trees again, or go to your search history.

 
From the Ancestry app:

It's not currently possible to search public family trees from the app. Try the steps above in a browser. 

Troubleshooting

Searching for a tree by the tree name: It’s not currently possible to search by tree name. If you want to see the same tree again, ask the tree owner to invite you to the tree or search again using the steps above. You may need to click View all to find it.

Copying multiple people from a tree: To copy multiple people, click on each person individually and copy them one by one.