source: https://github.com/ankitects/anki/tree/2.1.44/rslib/src/storage/upgrades

Changes are being made to the Anki database structure. According to the mod.rs file in the above link, the minimum supported version (and the version that new collections are created with) is version 11. This number is found in the ver field of the col table. AnkiDroid uses version 11 of the database at the time of this writing. Desktop Anki seems to perform upgrades to the database when opening it to improve performance, but then downgrades to allow compatibility with Anki clients still on version 11.

The schemaXX_upgrade.sql files indicate what each upgrade does. Here are explantions for each of those files:

schema17_upgrade.sql

Additional fields (db term, not Anki's fields) for the tags table.

schema15_upgrade.sql

New tables FIELDS, templates, notetypes, decks

schema14_upgrade.sql

New tables deck_config, config, and tags to replace the corresponding fields in the col table.

deck_config

• config - Contains a protobuf encoded blob

Theories

I almost exclusively review on AnkiDroid and so I'm on db version 11 and can't confirm these:

• new tables have a field of mtime_secs integer NOT NULL is this modified time in epoch seconds? Aka what the older tables have as mod?
• similarly, there's a new field of ntid integer NOT NULL, in some of the tables. Is this referring to the id of a note or the id of a note type?