The entries in EU business registries are public for all companies, not just publicly traded companies. You can check who owns how much of what company and you can also check their yearly reports. In most countries this comes at a small fee for the registry extract, but the information is readily available, in some countries it's even free. As an example, here's the Estonian business registry extract for the company Foobar OÜ (I'm not affiliated, I just searched for foobar) [0]. This is not a publicly traded company, yet you can see who owns the company in what fraction and if you download the latest annual report you can also see how much the company is worth.

This information is legally required to be available to prevent money laundering, fraud, other financial crimes, etc.

[0] https://ariregister.rik.ee/eng/company/12199449/Foobar-O%C3%...