I really don’t understand any of the responses here to the OP.
A sui juris Church is, by definition, self governing. If the “governing” see is sede vacante, an apostolic administrator is appointed. While they do not have the powers of a bishop, they do have authority to run the Chancery and ensure that the life of the Church continues, thus handling administration as required in connection with the sacraments (marriages, baptisms), etc. They generally cannot reassign priests or, in the case of the larger Churches, reassign bishops, nor can they promulgate changes to liturgical practices or discipline, etc. Everything runs as it had under the most recent bishop, and in accordance with establlished norms of particular law and canon law, more generally.
For example, before the current Major Archbishop of the UGCC was appointed (His Beatitude Sviatoslav Shevchuk), the governing see was administered by the current Metropolitan Archbishop of L’viv, Vladyka Ihor Vozniak, C.S.S.R. While he did not have the powers of Major Archbishop, he was appointed as apostolic administrator and was responsible to call the Synod that led to the nomination of the current Major Archbishop, according to the previously established norms of particular law for the UGCC.
The Pope is never directly in charge of a sui juris Church, just as he would never become the proper ordinary of any Latin diocese. In a Metropolitan Province of the Latin Church, the Metropolitan Archbishop automoatically becomes the administrator of a suffragen see in the case of a vacancy.
However, another hierarch can be assigned to oversee an Eparchy or Eparchies under certain extreme circumstances, as was the case for certain jurisdictions subject to Communist rule in Eastern Europe. In fact, the Bishop of Warsaw, later Blessed Pope John Paul II, served in such capacity for several Eparchies throughout Eastern Europe at one time.
The bishop (or in the case of a sede vacante see, the apostolic administrator) is accountable to the Holy See as per the CCEO, depending on the form of the Church (Patriarchate, Major Archiepiscopal, Metropolitan, etc.), yet is always the ordinary of their jurisdiction, subject to Canon Law and in communion with the Holy See.
