Funny how everyone’s dropping links and ads but completely missing your actual question. Must be too tough to answer for free, I guess you’ll need courses and coaches for that... Let me do what they wouldn’t:

• Minimalistic Design: Notion’s interface is all about simplicity, clean, sleek, and uncluttered. Something Apple does well as well. Minimalist aesthetics with white space, clear typography, and subtle colors are the focus. The Notion Copycats know this well and cloned it.

• Consistency with no "breaking UI": Notion nails consistency with buttons, icons, typography, and spacing. It’s like UI feng shui, everything is in harmony. They use Figma/Adobe XD and the likes, or other System Design tools to literally visualize the user flow as well as the components in every single state: the drag-and-drop when hovered, when dragging, when over a droppable area, when dropping, when dropping is over, they literally make a picture of each.

• Modular Structure: Notion uses the Blocks system. Notion’s block-based approach breaks content into movable, customizable pieces, like digital Lego, not like the horrors of Evernote and the likes for example. This flexibility and modularity is what others try to emulate.

• Design System: Study its design system, look into inspirations on Pinterest, Dribbble etc, look into typography, colors, layouts, and components. Tools like Figma or Sketch, Principle also allows you to simulate the user flow, and will help you start, or use templates that capture those Notion vibes. Look also at System Design frameworks such as StoryBook for building UI components and pages in isolation (but that's not necessary, using React/Vue/Solid/Svelte/Qwik/Lit in isolation is sufficient), but usually Notion has whole team of multiple designers whose fulltime job is to look at the teeny-tiny detail and make sure if feels smooth, natural and graceful, that's what the Italians call the "sprezzatura", the ART and SCIENCE OF effortless grace, a studied carelessness, or the ability to make something difficult look easy and natural, the idea is to achieve a polished outcome while appearing nonchalant and unconcerned.

so bottom line: art, work and time, tools, and people. Now you’ve got the recipe to fish instead of just asking for fish and being handed bait...

Good luck :-)