Is it possible to add other companies I own under the same user ID (i.e. same login email)? What would the process be and would I be able to switch between companies?

Warm welcome, @skb74. You can add or create other companies under the same sign-in info in QuickBooks Online.

While adding other companies under the same user ID in QuickBooks Online is possible, each company file has its subscription. 

If your other company doesn't have a subscription to QBO, you may visit the QuickBooks pricing page to view and select the subscription you want for your company file. 
 

On the other hand, if your additional company file is already subscribed to the QBO plan, you may use the same login credentials when logging in to navigate your QBO accounts easily. 
 

To add a new company:

1. Go to the Sign-in page and select Create an account. 
2. Select a plan, and when you reach the Sign-up page, click Add another company or add another company to the existing login.
3. Please follow the instructions on your screen to complete the setup process.

The next time you sign in, you will see the list of companies under your account. 

For more detailed information, check out this article: Create or add another company file to QuickBooks Online. 

On the other hand, I'll add these articles to help you manage your transactions in QuickBooks Online:

• Categorize online bank transactions.
• Match online bank transactions. 

I'm still around whenever you need further clarification about creating or adding a company with the same sign-in info. If you need to reach me, hit the reply button below. Have a pleasant day ahead.