A Daf Hayomi shiur that zooms nightly, and after the end of the shiur it is already the correct time for Sefira counting, is it correct that all join and count together? Most davened Maaariv before the correct time for Sefira.

on question 2721 and others we wrote if one learned the Masechta to be finished and is on his way to the siyum he could join the siyum via telephone or online. Therefore, if every participant in the siyum, eats his own seuda with his family and friends, it is considered a seudas mitzva. See also recent answer 4192 regarding a Siyum for an Avel on Erev Pesach via zoom. Therefore, although one cannot recite a brocho for someone else on line and the ones that hear the brocho should also not answer Amen, it is better to perform a mitzva at the same time that others do. It is also beneficial since they may be reminded to count Sefira.