when editing photos on my Mac, I just added a text box, no issue. Now, how do I make that text BOLD?

Follow the steps I provided above 🙂 Highlight the text within the text box and then click the B for bold. You can also refer to the official documentation - Use Markup to write or draw on a photo in Photos on Mac - Apple Support

Hello

Could you please elaborate on what application you're using?

If using Preview, click on the Aa

In Photos,

1. Click Edit (top right)
2. Click More (three dots)
3. Click Markup
4. Click Aa

I hope this helps!