Carry on info for United...(newbie travel ?)

I have a flight coming up and I haven't flown since I was a teenager. I'm trying to understand the carry on regulations. I will be making the flight with my DD and my stepdaughter and we have a layover. I would prefer not to have to check any bags.

I found on the website where it states:

United Airlines will permit one bag plus one personal item (see below), per customer to be carried on the aircraft.

The maximum combined linear measurement (length + width + height) of carry-on bags must not exceed 14 inches x 9 inches x 22 inches (23 x 35 x 56 cm) or 45 linear inches (114 cm).

Please note: All carry-on items must be stowed under the seat in front of you or in the overhead bin. Items may not be stowed in the seat back pocket. Any item in excess of carry-on baggage size or allowance requirements as listed above will be checked to your final destination and may be subject to applicable checked baggage fees.?

In addition to one carry-on item, you may bring one personal item such as a shoulder bag, backpack, laptop bag or an item of similar size.

The following items are not counted toward your one bag and one personal item limit:

• Overcoat or wrap
• Umbrella
• A reasonable amount of reading material
• Pet carrier (fees apply)
• A collapsible wheelchair

• Child restraint seat meeting FAA approval standards
• Camera
• Diaper bag
• A limited amount of duty-free merchandise or food purchased in the airport
• Assistive devices (a cane, one set of crutches, prescription medications and any medical devices needed to administer the medications, a portable oxygen concentrator (POC), etc.)  

So my question is, they are saying that ALL items you carry on must fit within the 45 linear inches or just the actual suitcase?

I think I read it to mean that if you carry on a smaller bag & a backpack that is fine but you can?t carry on a suitcase that meets the maximum 45 linear inches and a back pack. Is that correct? And from what I am reading you can still carry a camera separately (like I can just have my DSLR)  and a smaller bag and a backpack. I am planning on taking my laptop in a backpack and it?s my understanding I will need to pull it out when we go through security. Sorry if these are dumb questions but I am a newbie when it comes to flying. Any other tips, pointers for packing would be appreciated. I really feel lost when it comes flying.