For everyone on the network to agree on the same ledger chain, there are various rules about how the UNL should be formed. Specifically that there should be a minimum amount of overlap of people's UNLs. We won't get into that here.
The most important factor in choosing this UNL, is to make sure that you pick validators which are not colluding. There is a set of rules and best practices for validators to enforce this. The most important is that validators must be publicly identified. For example, if hypothetically Microsoft is running a validator then it should publish its public key in a way that certifies Microsoft is under control of the validator. An easy way to do this is to put a text file with the public key at the company's domain, reachable via HTTPS with a known certificate. For example at https://microsoft.com/ripple.txt
Looking at this masternode system of Dash, which is essentially the same as Ripple validators (with minor technical differences). But in Dash, the masternodes identity is secret. This is exactly the scenario that makes dash insecure!
One company and one company only sells all XRP = 100% Centralized
Something decentralized can not have missing data as it is the case with this garbage.
A service does not make it decentralized. Its like saying Walmart is decentralized because it has stores scattered all over. All Sales still congregate at one place from which sales tax is paid (or not).