How extra regressors are forecasted in fbprophet in cross-validation?

During validation of the model I have questions:
1. Having initial = '1000days' why in df_cv the first forecast point starts from 1005 point?
2. If I add extra regressor, how is it forecasted? I tried to use true values (which actually should not be used in cross-validation) and I tried to use the last true value from training set. But still I can't get the same yhat as in df_cv while making one separate forecast

P.S. I am not interested in 'for' loop, just wish to get an explanation
Thanks in advance!