Short answer
HubSpot's KB: in the updated editor, place fields and elements above, below or next to each other, up to three fields side by side. You can also add content between fields. Legacy forms follow a separate article.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
Check the form on mobile, where side-by-side fields stack.
3. Related
Multi-step forms and form styling.