Help me understand this... (Classes and self).

[Yoriz]

your code

tab_one = tk.Frame(nb) # <-- target actual # #

is making a new frame who's parent is the notebook that has a pointer to it in the variable nb.
this new frame has a pointer to it stored in tab_one which is a local variable to the method build_tab.
when the method build_tab has done all its code, tab_one will be gone leaving no access to the frame.

by changing it to

self.tab_one = tk.Frame(nb) # <-- target actual # #

tab_one becomes a instance variable, that is accessible from both inside the instance(by other methods) and outside the instance(by the object instance).

Example

class MainFrame:
    def __init__(self):
        self.build_nb()
        
    def build_nb(self):
        tab_one = '<local variable>)'
        self.tab_one = '<instance variable>'
        
        print('printed by MainFrame>build_nb:\n'
              f'tab_one = {tab_one}\n'
              f'self.tab_one = {self.tab_one}\n')
        
    def output_tab_one(self):
        print('printed by MainFrame>output_tab_one:\n'
              f'tab_one = tab_one cannot be accessed its forgotten about\n'
              f'self.tab_one = {self.tab_one}\n')
        

main_frame = MainFrame()
main_frame.output_tab_one()
print('printed by instance of MainFrame:\n'
    f'tab_one = tab_one cannot be accessed its forgotten about\n'
    f'self.tab_one = {main_frame.tab_one}')

Output:
printed by MainFrame>build_nb:
tab_one = <local variable>)
self.tab_one = <instance variable>

printed by MainFrame>output_tab_one:
tab_one = tab_one cannot be accessed its forgotten about
self.tab_one = <instance variable>

printed by instance of MainFrame:
tab_one = tab_one cannot be accessed its forgotten about
self.tab_one = <instance variable>