A quick heads up, on Schufa, I did a project involving schufa checks bit from a (b2b ) business to business perspective back in 2008: ie the overall scoring process and the part that Schufa plays.
1) simply put - Schufa gathers “at least” a list of all financial transactions by you for those where defaulted payments were reported. The US equivalent is called FICO. In China it is a government function and UK , Australia, Canada use multiples (even more intransparent scoring methods) like Equifax etc. You get added automatically "for free
" to Schufa for tracking, when any business checks your credit score, or you signed any contract in your life with Schufa clearance. Even billionaires have electricity bills, water, telephone etc. contracts where Schufa checks are standard.
2) If some business “believes” that you defaulted on them, they can report you as individual (using your credentials) and that “misdemeanor” gets added to your list, even if it might be contentious, like a bill not paid due to insufficient services, or a landlord freezing you out in your flat in winter, or you defaulting on your home loan for 3 months, after losing your job and still repaying it afterwards. There is no higher instance to complain than schufa itself. Schufa has a process to contest the listing , but that involves you writing an excuse letter to the business that raised the request and begging for mercy.
3) the Schufa credit score does not only involve your financial history, in fact demographics affects the score , “slightly”: where you live, your income, criminal record, your family members score and financial security, friends scores- relations to dodgy individuals etc.
4) Every business that have your credentials and proof of imminent contract can request a schufa score if you placed a clearance checkmark in contract, to secure themselves and be sure that you are a model payer.
5) Even the number of Schufa requests in a short time period lowers your score over time, so rather request it yourself- you are entitled to a free disclosure on the first time that contains your score. Schufa however formally “prohibits” you from sharing that, because they also want to know who is asking questions about you and persist that forever. A credit scoring report for 29,95€ contains a list of all business that reported you in your lifetime on this planet for some reason.
6 ) next time you sign any contract, even registering with an online business with account details or borrow money from any bank, don’t forget that likely all of that is shared and remembered for “all” future businesses where you have financial dealings later in life.
7 ) government regulation (§ 15 DS-GVO) have tried to mitigate personal individual fates by allowing certain entries to be removed if you can proof “repayment” but only if it was below 2K€. Outside EU there is none such mitigation. There is also a movement in Germany to get transparency called openSCHUFA to try to reverse engineer the logic by having people all share their scores to the platform, but it is Sisyphus task, since the algorithm is modified constantly also using AI and a trade secret, all data sources is not the report ( ie Schufa is buying Your Facebook, Instagram, Whatsapp, Twitter profile scores already a decade ago). And that, that is why Schufa is trusted : because of its high profile accuracy… good luck now on maintaining a good and clean score, keeping good online relations (by unfriending & blocking) and avoiding “hitwords” that reflect bad on your social intelligence. GDPR provides a right to privacy and the right to have some things forgotten, but it still does not apply to your financial paper trail, to date.