A user interface needn't be all visual (e.g., in a device with physical buttons, the buttons are part of the UI).
Some visual things might not be best thought of as part of the UI (e.g., if you have a piece of software for making publication-quality graphs, and when you ask it to plot a smooth curve through some points it does a lousy job of picking the smooth curve you actually wanted, arguably that's a visual matter but it isn't really part of the user interface, it's about the algorithms underneath).
It's not really how the work is split in bigger commercial teams. The folk fixing the layout bugs aren't the folk who designed the page aren't the folk who designed the flow. I like to avoid big teams but even for me UI/UX mean very different things, you can find this split even on relatively small teams
A 2-3 person team might have frontend/UI/UX being the single 'frontend developer'
A 5-7 person team might have one or more frontend people and one UI/UX person living in Adobe suite all day
Maybe the 7 person team is receiving storyboards from a parent team where all the UX work was done centrall across several 5-7 person teams.
A user interface needn't be all visual (e.g., in a device with physical buttons, the buttons are part of the UI).
Some visual things might not be best thought of as part of the UI (e.g., if you have a piece of software for making publication-quality graphs, and when you ask it to plot a smooth curve through some points it does a lousy job of picking the smooth curve you actually wanted, arguably that's a visual matter but it isn't really part of the user interface, it's about the algorithms underneath).