From what I understand.
At all speeds the electric motors drive the wheels, either in series or parallel to maximize efficiency. (Series for lower speeds, parallel for highway speeds). At speeds above 45 mph, the engine can engage the single gear to power the wheels directly. I do know that the car does in fact have transmission fluid. Whether that's used as a cooling aid, and/or clutch activation, I'm not 100% sure.
I hope that helps. From a traditional standpoint, there is no transmission between the ICE and the wheels, just a single fixed gear.
At least that's my basic understanding of the system.
I found this much better explanation when searching google for Honda 2 motor hybrid system.
2018 Honda Accord Hybrid features third-generation MMD two-motor system; no heavy rare-earth metals in motors
An excerpt from that article:
Drive Force Transfer. Just like the previous model, the Accord Hybrid is not equipped with a conventional mechanical transmission. Instead, motive force transfer is accomplished through the interaction of the Accord Hybrid’s gasoline engine and two electric motors. Coordinated by the IPU, this form of drive force transfer offers smooth and predictable acceleration matched with efficient low-rpm highway cruising when the gasoline engine is in operation. Gasoline engine shutdown is seamlessly integrated into the operation of the Accord Hybrid when appropriate.
The drive force transfer system operates without the need for a torque converter, mechanical pulley or belt. It instead uses two motors for driving and generating power. The system is optimally and rapidly able to control both engine and electric motor rotation in order to deliver higher fuel efficiency and quicker engine response in each driving mode.
When cruising at mid- or high-speeds in the high-efficiency range of the engine, a lock-up clutch is engaged, connecting the drive motor to the generator motor to transmit engine torque directly to the drive wheels as efficiently as possible. In EV Drive operation, when the battery-powered drive motor is used for either acceleration or regenerative braking, a clutch disengages the stopped gasoline engine from the drivetrain to eliminate efficiency loss from mechanical friction in the engine.
Improving efficiency and reducing weight and size, the drive force transfer system of the second-generation i-MMD integrates the torque limiter within the flywheel.
And yes, I know it's about the accord hybrid, but it's the same system used in the 3rd Generation Insight.