As documented by Jeffrey S. Lee, the AdLib simply provides raw programmatic access to its OPL2:
The sound card is programmed by sending data to its internal registers via its two I/O ports: ...
The sound card possesses an array of two hundred forty-four registers; to write to a particular register, send the register number (01-F5) to the address port, and the desired value to the data port.
After writing to the register port, you must wait twelve cycles before sending the data; after writing the data, eighty-four cycles must elapse before any other sound card operation may be performed.
So, no MIDI, no other high-level format. The card produces nine channels of sound, each of which is the product of two sine-derived functions; you can instead configure it as six of those channels plus five percussion channels.
It supports automatic application of ADSR but otherwise it is a simple modal device. Set the current instrument set, their frequencies and volume envelopes, then they'll play continuously until you tell the card otherwise.
So, unlike MIDI or other formats like it, there's no inherent sequencing or timing of notes — the card has no autonomy in proceeding through music. It just makes the noises you've currently assigned to it.
(and as to implementation of that expensive-sounding audio generation, see this reverse engineering; summary: it's all log tables)