They were in on it from the beginning. The way FOX refused to call Florida for Trump even when 96% of the vote was in, but called Virginia for Biden when only 3% of the vote was in made it obvious: they were under orders to present it such that Biden was ahead every inch of the way.
That they made it clear they weren't going to count the mail-in votes until the in-person ones were cast was also very obvious: "We can't count the mail-in votes until we know how many we need to fabricate!"
If the Republican leadership had protested vociferously it could have stopped the steal or made it much harder