Invoicing in the app is a little bit different from the web version, userkunal-lexicon.
I'll guide you through the steps on how to apply itemwise discount using the QBO mobile app. Here's how:
Here's an article about this for more details: Discount By Line Item On Invoices And Sales Receipts.
Feel free to check out these articles as well about using the app for more details and reference:
Keep me posted if you need more help or when looking for something in the program.