What things exist?

See Cosmos, Cosmos@TheBrain, Data & Knowledge, Cosmography, etc.

George P. Lakoff - All Reality Consists of What Things? (Closer to Truth)

Contemporary ontology

1. What are the most general features of the World, and what sorts of things does it contain? What is the World like?
2. Why does a World exist – and, more specifically, why is there a World having the features and the content described in the answer to question 1?
3. What is our place in the World? How do we humans beings fit into it?

See Metaphysics.

I think the answer to these questions lies in Systems theory, Mathematics, and Science.

Sciencewise ontology

Concepts

Entity

Property

Process

Structure

Conceptualism Conceptualism is a philosophical theory that explains universality of particulars as conceptualized frameworks situated within the thinking mind.

https://www.wikiwand.com/en/Categories_(Aristotle)

Is Anything Real?

https://medium.com/fathominfo/a-visual-inventory-of-wikipedia-fc42afd2bcdb

See Cosmos, Cosmos@TheBrain, Data & Knowledge