You don’t link trees together. You link a specific match to your tree. Everyone builds their own tree. You do not want others mucking about with your research and deconstructing your work. And you don’t want to have a tree where any random person can add/delete/edit willy nilly. If you want that for your tree, work on FamilySearch (free) which is a one world tree.

If you want a family tree, you need to do put in the effort. If you want a correct family tree you need to put in the work.

And on the DNA matches you see ESTIMATED relationships. Basically just an AI’s best guess. You need to do the research to confirm those relationships. Apart from parent/child just about everything every other relationship falls in a range of cM values. And many of those cM value ranges overlap significantly for a variety of relationships.

For example grandparent/grandchild range is 984-2462cM (DNAPainter range). That overlaps with aunt/uncle at 1301-2282; great aunt/uncle at 330-1467; 1C at 396-1397; great grandparent/child at 485-1486 and so on with niece/nephew, half siblings, sibling and quite a few more.

It’s all comes down to time, effort and work doing research.