Hi James,
You could add a new field for contact entity in modern editor:
1. Create a new field for contact entity.
1.1 Open https://make.powerapps.com/
1.2 Switch the environment to your marketing instance,(1)
open Data -> Entities,(2) choose Contact(3).

1.3 Click + Add field:

1.4 Create an option set field for contact type:

1.5 Save change:

2. Add the field to Contact form
2.1 Back to Contact entity,
switch to Forms tab,(1)
open Contact (Main) form.(2)

2.2 Opeb Tree view, switch to Details tab

2.3 Switch back to Fields, drag the custom field to section you want.

2.4 Save then Publish

3. See result in Contact -> contact form in corrensponding tab and section:

4. Build segment based on the field:

5. Now you can add these segments into customer journey for marketing campaign.
In addition, you might had found that icons were missing at upper right corner at top bar,

if so, you could open settings UI directly by input URL below:
https://<org>.crm.dynamics.com/main.aspx?settingsonly=true
then you'll enter traditional settings UI, and do customization too with Stephani's links.
Regards,
Clofly