How do tax offices even know about my income ?

Hello maybe a dumb question but I always wondered how does the tax department of each country is even able to keep track of messy situations ?

For example let's say this year I am making around 1k euros on paypal each month online with a fully remote business. From january to march I live with with my family in Belgium. Then I found a regular job in Luxembourg but decided to live in Germany for cheaper rent from april to July. Finally I decided to rent an appartement in Luxembourg for the rest of the year.

In this situation we have - 3 countries. - No clear fiscal residency since I never stayed more than 6 months in a country. - Two income streams. - Income earned in the 3 countries

How do peoples even manage this kind of mess ? Do they just report randomly in one country and pray ? Pay crazy amounts to a tax expert ? How could governements know about all this ? I feel like there are so many frauds that can easily happen, it seems absolutely impossible for a governement to keep track of income streams for the whole population in each country.