Howdy! I use the PC version of medibang and while the steps are similar this is how I usually go about things
So as I'm sure you might know you'll go to Layer > Add panel material to get the starting panel borders
choosing your line width and border color from the popup
of course after that you get that square/rectangle you were talking about
as for diving it into the panels you want there's a few different methods:
-one would be to use the Layer > Material > Divide options which lets you to break the existing panel equally
after you select what you want you'll get the resulting panels like here:
for more manual panel editing you can use either of these tools (in the red)
the slice tool lets you chop up panels wherever either vertically, horizontally or diagonally so the result looks like this
if you want to made other edits you'll use the operation tool (the little arrow hitting the pentagon)
i've only ever used it to resize my panels so i can't give much more insight to what the other buttons mean but i remember Medibang has a tutorial on their help page here:
Hope this stuff helps