I will record our inventory purchase and other office expenses. Can you help me how to record these transactions? There are two options: Category Details and Item List?

Glad to have you here in the Community forum, @lim-charissadani.

I'd be delighted to assist you with keeping track of your transactions in QuickBooks Online (QBO).

You can generate a Bill, Check, or Expense transaction when tracking inventory purchases or other office expenses. 

You can use the Item details area to pick specific products and services to record inventory purchases. Then, when recording other office expenses, you can select an expense account to track expense transactions from the Category details area. 

For more details on managing your expenses, see this article: Enter and manage expenses in QuickBooks Online.

For future help, here's a great resource for your guide in recording bill payments: Enter bills and record bill payments in QuickBooks Online.

Should you have any other concerns, let me know and I'll help out again. Have a great day.