If you have a main site then ensure its first page is always index.htm(l).
Sub-sites need not have this luxury and can be published into a sub-folder (e.g., subfolder01).

Linking from the main site would have a URL of "subfolder01/page01.htm(l)", assuming you know your structure, you do not need the full URL.
Sub-site to sub-site links are a little trickier. To got to Page 5 on the third sub-site from another sub-site would require "../subfolder03/page05.htm(l)"; the ".." means go up a level (to main) and navigate from there.

This is why I ignore subfolders and publish everything into one level, ensuring no page filename is duplicated. Far simpler as all links are then "page01.htm(l)", "page05.htm(l)", ...
... the ",htm(l)" part is essential as Link to only will display those pages in the current design.

Acorn