Stock Prediction using previous data

Question

Hello, I had followed one of the videos on youtube for making stock predictions. It works well, however how do you now make it predict into the future? It worked from taking data from 2012-2019, then predicting and comparing it to 2020, but, how now do you take the data from 2012-2020 and predict for 2021, since you cannot compare it to 2021 since that data obviously doesn't exist yet?

Its easy enough to get the data to train for 2012-2020, thats not the issue, that actual issue I'm having is making the prediction. Can someone help me out?

(Note: I'm just using this to learn, now to actually make stock purchases. I just find looking at stock prices and numbers easier for learning compared to other ways)