Even though basic (or non-smart) trainers lack the built-in technology to simulate resistance like a smart trainer, they are still compatible with indoor cycling apps with the use of a speed sensor. The only difference is you control the resistance.
To make any basic trainer smart compatible, all you need is a speed sensor on your bike and you are ready to ride.
By connecting with a speed sensor, you are able to get speed readings as well as power data from most apps (power is based off the known power curve of each trainer and calculated using your current speed).
Though basic trainers require, at minimum, a speed sensor to connect, you can also use a power meter to connect to your training app of choice.