Here are some ideas to consider as you research the subject...
Rename the form, subforms, NavControl, Nav Buttons in a way to help you understand what each object is.
Get familiar with each control's properties and their limitations (Buttons, Tabs, Etc.)
Expect limitations when using VBA. Here is syntax to set value of an unbound textbox within Nav Control
Code:
Forms!frmNavForm1.NavigationSubform.Form.txtSample.Value = 12
However this will not work.
Code:
Forms!frmNavForm1.NavigationSubform.Form.txtSample.SetFocus
My experience is that Navigation Controls do not like VBA and prefer Macros.
You can use VBA to get stuff done and set properties
Docmd.BrowseTo
ObjectName:
PathToSubformControl:
WhereCondition:
It may be better to create Macros. The Nav Control's limitations seem to be on par with limitations of Macros
https://www.youtube.com/watch?v=A0HowZInuaI