One, free action for Photoshop
This action isolates real drawn or painted media & marks, retaining their original opacity and making the paper transparent.
⚙️ How to use it
With your scan on its own layer, press play on the action. You'll be prompted to adjust the levels. Make sure the paper / surface is pure white and then press ok. Photoshop will do the rest.
Note: This action works best with original content that is in greyscale / black and white. You can then make it whatever colour you want!
You'll get one action: 'Isolate'.