Thank you a lot! The difference between setup() and loop() was a bit obscure to me but now I understand more how each works, the importance of writing in the right function.
The code still didn 't work _ the Oled now shows "BPM: D"_ D is the beginning of the text (Date,Time,BPM)
I did the tests you put as an alternative and "ABC" can be seen except for variable-type String.
But I looked at the tutorial, the explainations are really clear,so well illustrated; the variable part or String, array for example, I get the essence of it, I just need to read it more than twice to understand but it definitly make the code more accessible.
So maybe I am gonna try to manually type in the variables by using The Arduino String Object shown in the tutorial; putting aside the SD card..
Thank you so much for helping me out with this code and making me understand how the programming works !