First of all, you may go to read a document from the Library. And then tap "Split View" to choose "Current Doc & Notes".
Second, please follow the next steps to swap the note section to the left side.
Step 1: Please kindly tap the hidden menu icon in the red as below.
Step 2: Please kindly tap the icon as the arrow in red as below.
Step 3 & 4: Please kindly click the icon as "Step 3" instructs and then choose the "Swap" option when the drop box shows up.
Then you can see that you swap the note section to the left side as below.