You will find the links page for any indicator page at ...{YourOrg}.opal3.com/links/{id}, or by clicking on the "links" slider button on an indicator's setup/details page:

When adding links to other indicators, note that the first options that then appear are those indicators that are in the same organisational structure unit as the indicator you're working from. If you want to link to any other indicators, just change the filter setting at the top right of the screen.

In a hierarchy of linked indicators, the update page of higher level indicators will enable reviewing the results of lower level indictors.

For roll-up calculations see this article.