Python Tkinter changing UI from the main thread VS new thread

In my learning python process, I came across the typical freezing problem when main thread is blocked by a long running function. The solution I found was basically starting a new thread. I ended up having 2 ways that seemed to be working correctly, but I would love to know if my understanding is correct.

The first way I believe the pb.destory() is an example of updating UI from a non-main thread

# --- functions ---
def longFunction():
    for i in range(10):
        print(i)
        time.sleep(0.25)
    # I believe this is updating the UI from the non-main thread (remove the progress bar)
    pb.destroy()
    
        
def createNewThread():
    # I am expecting the prgoressbar animation to not get blocked/freeze.
    pb.start(10)
    pb.pack()
    
    thread = threading.Thread(target=longFunction)
    thread.daemon = True
    thread.start()

root = tk.Tk()
pb = Progressbar(root,mode='indeterminate') 
tk.Button(root,text='START', command=createNewThread).pack()

root.mainloop()

The second way, I basically tried to use after() with a flag that I checks on periodically. I believe in this one the UI update happens in the main thread

# --- functions ---
def longFunction():
    for i in range(10):
        print(i)
        time.sleep(0.25)
    global isDone
    isDone = True

# Check on isDone flag every 100 ms 
def checkFlag():
    global isDone
    if isDone:
        # I believe this is updating the UI from the main thread (remove the progress bar)
        pb.destroy()
    else:
        root.after(100,checkFlag)
         
def createNewThread():
    # I am expecting the prgoressbar animation to not get blocked/freeze.
    pb.start(10)
    pb.pack()
    
    thread = threading.Thread(target=longFunction)
    thread.daemon = True
    thread.start()
    
    root.after(100,checkFlag)

# flag
isDone = False
root = tk.Tk()
pb = Progressbar(root,mode='indeterminate') 
tk.Button(root,text='START', command=createNewThread).pack()

root.mainloop()

My questions are:

1- Are my assumptions correct where the first way is updating the UI from non-main thread and the 2nd way is updating the UI from the main thread?

2- Is there an issue in updating the UI from a non-main thread ( I know in WPF/C# it causes too many problems)

3- Is there a better solution?