If we told you we would have to hug you. Sorry

Short answer: in the context of filename obfuscation, indexers do not do any indexing
When making and posting a filename-obfuscated post, the uploader makes his own NZB and shares the NZB with one or more indexers

Indexing:
Find the yEncode specification. It's a very short text file. Binary files are split into segments smaller than 1m bytes, because Usenet has always been a text messaging platform, and 1m bytes is a common maximum for message size. Each segment is yEncoded, and a Usenet message is created - the yEncoded binary chunk is the message body, and the body is appended to a Usenet message header. The yEncode spec defines a format for the Subject line to contain the filename. All the thousands of segments (as Usenet articles, or messages) are posted to the same newsgroup

Indexing reads the article headers from one or more chosen newsgroups, scans the headers for filenames, and builds a NZB using the article headers' Message-IDs
Use the Web UI at binsearch.info to view in an arbitrary newsgroup a list of articles matching a specific filename
Binsearch can dynamically create a NZB from the articles you select from its Web list

Indexing is trivial. Anybody can choose a newsgroup and index the binary files it contains within its articles. That's a problem, because copyright trolls use indexing to collate a list of articles, and send that list of Message-IDs to Usenet providers in the form of a takedown demand

The first thing I thought about was that if headers contained the title and other easily identifiable information there shouldn't be much thought to choosing an indexer

In the indexing context, the user's choice of indexer depends on whether all indexers are indexing the same groups. Or if Binsearch covers the groups you need, use it as your indexer

In the obfuscation context, join the indexer which has properly obfuscated posts which contain the files you want to download

Obfuscation:
Over several years, uploaders and a couple of indexers collaborated to develop a filename obfuscation method which can resist copyright takedown demands. Obviously, this means removing the filename from the Subject field of the message header. After about 4 iterations they settled on the method used by many uploaders today

No filename, no more indexing by filename matching

An uploader knows all the Message-IDs of all the articles he posts. He has all the information necessary to make a NZB file for his own uploads. If an uploader creates his own NZB, he can send it to his favorite indexers

If you're a member of that indexer, you can download that NZB. If you're not a member, you miss out - unless someone who is a member shares the NZB outside the indexer

Minor details:
The PAR2 files are created using the original filenames
The PAR2 mechanism includes a table of hashes (magic cryptography) which are used for verification and repair
The downloader app (or the downloader using his favorite PAR2 app) invokes a PAR2 repair on the downloaded filename-obfuscated files. The PAR2 software hashes the files, matches the hashes stored in the PAR2, and uses the original filenames stored in the PAR2 to rename the file to original

There is no direct lookup from obfuscated filename to original filename. If you read someone claiming indexers are able to deobfuscate, do not believe that

In the best obfuscation method (some inferior methods are still in use), the articles are spread arbitrarily across 6 to 10 different newsgroups. The idea that articles are contained inside newsgroups is an illusion. An article has a globally unique Message-ID. It is not necessary to tell the server which newsgroup the article is in. The Message-ID is all that's needed. Of course, by not being in a single newsgroup, the articles can't be indexed

Because they embed the original filenames, the PAR2 files are posted to different newsgroups

Back in the 1990s, this was all done manually, one would hope someone has figured out how to automate it by now. At the time, some sites even compensated users with additional access time for manually sifting through posts and piecing together posts and par files for nzbs.

The nzb file contains this information. That is partly why you download the nzb from the indexer.