Online Personal Training takes different formats.
At the most basic level you will be provided a workout programme - hopefully after completing some paperwork about your health status and so on, you know... it helps if the person knows what the goals are you are working to so that they should be aiming the programme towards that.
Ideally the programme would also be progressive. Not the same exercises, sets, reps and weights for the duration.
I know some people just download a programme to work to. They reason that if the routine worked for someone else then it will work for them too. Whilst that could be right, it could also be very wrong. (Don't get me started on some of the strange exercises I have seen performed in gyms and when I have questioned the person the answer was "This guy/girl on YouTube swears by it and they look superfit")
Simplest form of online training then = a programme, usually with no accountability.
Next you have Apps. Yes actual apps. I have one. Look!