Timed Entry for Vatican, Vatican Museums and Sistine Chapel

If you buy timed-entry tickets, you will be standing in a line with all the other people who have tickets for that same timed-entry. To be honest, there really is no such thing as "skip the line" tickets. Timed-entry tickets mean you skip the ticket buying line which is the longest line at the Museums (and at most sites). Everyone will queue up to get in and then queue up for security. They really need to come up with a better name for the ticket selling process, since, when people read "skip the line", they assume there will be no lines anywhere.

There is no ticket required for St. Peter's itself, but the security line is usually very long. The only way to avoid it (which usually works, but no guarantee) is to pay for a commercial tour (probably over €100 per person) that covers both the Vatican Museums and St. Peter's. Those tours are normally allowed to take their clients through a connecting door that isn't open to the public. The disadvantage of that approach (other than the cost) is that you must accompany the guide through the connecting door. If there were parts of the huge Vatican Museums where you wanted to spend more time than the tour did, you will not be able to do that without losing your opportunity to use the connecting door to St. Peter's.

Just to add...

The Sistine Chapel is included as part of the Vatican Museum, you follow a path through the museum, and one of the last rooms is the Sistine Chapel. For the museum, yes, you can get timed tickets, purchase well before, they sell out fast. You could also purchase a tour through a third party operator, or one of the tour options from the Vatican, otherwise your only option is to stand in line and get in as available.

For St. Peters, no cost (except to climb the dome), and everyone stands in line for security, but it moves quick.

To be honest, there really is no such thing as "skip the line"
tickets. Timed-entry tickets mean you skip the ticket buying line
which is the longest line at the Museums (and at most sites). Everyone
will queue up to get in and then queue up for security.

Bingo. "Skip the line" is a marketing ploy to make you think you need to pay more for their product than someone just paying for a timed-entry ticket. Timed-entry IS "skip the (ticket) line". No one bypasses security-check queues where those are in place.

The ability to avoid the line-up for St Peter's by booking a commercial tour of the museums that includes it, as acraven has pointed out, is an exception where a unique benefit applies....except on Wednesdays or other days where special services in the basilica prevent use of the interior passage.

Not all tours can go from the Sistine Chapel into St.. Peter’s so if you plan on this, be sure your tour does

It is important to understand that there is no such thing as a 'skip the line' tickets so you don't embarrass yourself by trying to bust the line of those patiently waiting for their time slot. I have observed an American tourist do this, march to the head of a museum line, declare he needs to get by as he has a 'skip the line' ticket, be turned away by the security guard and then be hooted to the back of the line. Before COVID those with timed tickets often could walk in many places because lots of people didn't get such tickets, but now everyone has a time slot and you line up with your time slot -- and now there are almost always lines for everything. It is not as bad as the two hour line you might have back in the day without a ticket, but it can certainly be 15 or 20 minutes.