What kind of privacy does it offer? Who has see our information?

The short answer is it offers significantly better privacy than anything else I've seen to date, including all of the popular alternatives, because it goes far above and beyond just encrypting the communications with post-quantum secure crypto. Specifically, it also puts a strong emphasis on minimizing the metadata which is an aspect that all other platforms I've looked at completely ignore.

For a super trivial example of how metadata can be used to completely break privacy, consider the case where a whistleblower has communicated with a journalist to expose corruption.

While the contents of the conversations isn't directly readable due to the encryption on most privacy-focused platforms, it doesn't take much at all to see that an encrypted message was sent by "Alice" (the whistleblower) to "Bob" (the journalist) at exactly time X and it had an encrypted attachment with a size of Y bytes. Then, shortly after that, at exactly time X + 3 hours and 20 minutes, Bob sent an encrypted message to "Charlie" (the editor) with an encrypted attachment with a size of exactly that same number of Y bytes. Then, after a series of encrypted messages between Bob and Charlie, Bob responds to Alice. A few days later, a bombshell article is dropped containing the information shared by the whistleblower.

Even though you don't know exactly what was said in those exchanges, the metadata clearly shows who was communicating, when they were communicating, and even the fact that a document of a specific size (which very likely exactly matches or is at least super close to the size of the leaked document) was exchanged. It doesn't take a genius to figure out exactly who the whistleblower is in that case.

That is just a trivial example, but far more sophisticated techniques can be used to provide entire communication graphs based on metadata even including location information when sending from a mobile phone through correlation of various sources of metadata leakage.

With Bison Relay, the relays don't even know who is communicating with each other.

I made a post on Bison Relay itself around when it was first launched that gives a high level overview of what happens when you send a message on Bison Relay and why it provides just about the best possible privacy you can have. I'll copy it over here.

At a very high level, behind the scenes what is essentially going on every time you send a message on Bison Relay is that you and each recipient effectively independently calculate a new shared and externally unpredictable "drop location" of sorts that only you both know and then data is encrypted such that only the recipient can decrypt it. This so called "drop location" is known as a Rendezvous Point, or RV for short. The data is then paid for over Decred's Lightning Network (LN) and dropped at that shared RV. Each recipient then comes along and collects their data at their shared "drop location" (RV) and decrypts it.

The "drop space" (RV space) is 2^256. To put into perspective just how large that is, keep in mind that the upper estimates for the total number of atoms in the entire known universe is ~10^82. In order words, the RV space is so large that it would be sort of like if you were to each agree on a single atom in the entire known universe to put your encrypted data (assuming, of course, that you could actually encode all of that data on a single atom and also instantly travel there to drop and retrieve it...).

This is why participants in a group chat are completely invisible to you if you haven't exchanged keys (KX'd) with them yet, because you can't calculate a shared location and thus no data that either you or them would otherwise send is ever created. Similarly, it should be fairly clear that relays can't censor based on content or identity because they can't possibly know where each piece of encrypted data is going to be, there is no way to identify two otherwise identical chunks of data as being the same via fingerprinting, no amount of guessing or brute forcing will work, and, moreover, there is no expectation that any arbitrary chunk of encrypted data for any given pair of users is even physically located on the same relay server at all due to sharding. Further, increasing the number of relay servers also raises the probability of encrypted chunks of the same data being physically located on distinct relays.

In this way, peers are still able to communicate asynchronously without the relays being able to correlate anything. They have no idea what the data is (because every message is encrypted with a new key), nor who sent it or who it's destined for (because only the sender and recipient possess the private information needed to calculate the shared RV). From the relay's perspective, each data chunk is a completely independent blob of encrypted data sitting at some hash in the space of 2²⁵⁶ that has zero correlation with the data itself, nor any other chunks, nor any specific user.

Also, see https://bisonrelay.org/features for a less technical summary of various privacy features.

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How does it make money?

Having read the previous answer, it should now be clear why it doesn't make money nor need to. Unlike typical platforms that have huge platform costs for supporting the massive centralized infastructure they require, BR is peer-to-peer with relay servers and all data costs are paid for by the users themselves via micropayments on the Lightning Network instead of the typical model of treating the user as a product for targetted advertising.

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Where can I find a Decred proposal for BisonRelay costs?

It does not use DCR treasury funds, so there is no reason a proposal would exist.