I've read about many skip the line tickets- but someone told me- they only allow you to skip the ticketing line- not the security line, is this correct?
Security is usually mandated by law or by the property owner, a ticket will not get you out of a security screening line
Now, to warn you, most of those tickets are very unreasonably priced at third-parties as tourist scams or straight out scams (by giving you invalid tickets...)
This applies both online and on the site itself, don't be fooled.
Try to always scavenge the internet for the official venue site (like for the Sistine Chapel, the official site is https://www.museivaticani.va), you'll have a guarentee of the validity of the ticket.
The main issue with the official site is that it is usually where the tickets are sold out the fastest... For some sights, like the Sistine Chapel, the tickets are sold-out months in advance.
If you really want to get a last-minute ticket (without guarantee of validity), you can expect to pay from 2x to 5x the official price (there is a 3rd party site that I won't name that is selling last-minute tickets at €45 instead of the official price of €17, and this is far from the worst offenders (70-80 € pp. is not unheard of))
This same third party is also trying to sell you the Rome Tourist Card, this is also a tourist scam, and you are extremely warned to avoid it.
The only official tourist pass is the Roma Pass, is of course less full of sights but costs way less and also has full public transport (now, you may want to make your calculations to see how worth it, it is for you)