See in bp-templates/bp-legacy/buddypress/members/single
and read here for some general guidance. Note: even if only Twenty theme are described as example, you can use (more or less) the same technique for any other theme.
What ever, all starts by creating a child-theme as very first step.
Related to your question, do drop down list of the sub nav and hide them in the object nav fields, i think you don’t need to alter a template, but to write a custom function.
As i can be wrong about you goal, perhaps take also a look to core file bp-core/bp-core-buddybar.php to understand how nav and subnav are instanciated.
Read also https://codex.buddypress.org/developer/navigation-api/
