Besides the Opensolr calls, the app talks straight to the phone's own index, over HTTPS, with the index's own credentials. These are plain Solr requests.
POST /selectA typed search on words only, its facets, the filter lists, the groups of similar photos and their photos, the photos the map draws, and the quick checks: how many documents the index holds at the start of a sync, and whether it carries the photo schema. Also the one read that fills the phone's copy at install, 10,000 documents per page with a cursor.
POST /suggestAutocomplete, from the second character you type. See autocomplete and Did you mean.
GET /opensolr-photos-configThe version of the configuration the index runs, compared with the app's own at the start of a sync. See a new index configuration.
POST /updateDeleting the photos that left the phone or that you deleted, 500 ids per request; emptying the index for a Reset; and the commit at the end of a sync.
A search by meaning does not come here: it goes through photos_select, which runs it on this same index. Documents are written by Opensolr, through photos_ingest, photos_words and photos_restore, never by the phone.
The index refused the password the phone holds. The app reads the current one with get_core_info and repeats the request once.
The index is over its disk space or bandwidth. The sync stops and a notification opens your Opensolr account.
The index is not there. The next sync checks the account's index list and creates it again only if it is certainly missing.