Typically the best / simplest way for sustainable mostly passive income is rental property. Lots of how to guides out there. Mortgage rates are really low at the mo if you want to leverage.
I assume renting out would only be worse with the additional risk of having bad tenants.
This helped a lot https://www.khanacademy.org/economics-finance-domain/core-fi...
Here in Finland, the status quo seems to be that you obviously save for your own apartment as the first thing. From purely financial gains perspective, I couldn't find the rationale.
For example, in Belgium, stamp duty (a transfer tax) upon acquisition of a property is 10%. It takes a long while before you generate that back in rental yields.