Well there's 2 different items that can be used as inserts but only one that's physically designed to be a passive (allow wetness to pass through while adding anywhere from a few ounces additional absorbency to 20+ ounces additional capacity) which is called a "booster pad". The other looks identical but has a waterproof outer shell (just like a diaper) that won't allow anything to pass through it and these are called "liners" and are generally worn alone in place of a diaper within a pair of fixation pants or even reg underwear for those with light incontinence, urinary only.
I have tremendous Urinary IC when asleep (my body never reduces urine output along with a billion other issues with it) and as I age and my back degrades further I need some form of protection 95% of the time with the only time I can go without is when it is hot & humid (I do as well as I ever will from June-early Sept, rest of the year is a coin flip rigged against me in a BIG way) AND I know I can whip it out and take a leak asap, such as when fishing in a tournament or out on a quad ride. Other than that, it's a minimum of incontinence underwear with PUL waterproof pants over the top *for minimal leakage/able to go to the bathroom immediately upon sensing I have to go* though that's a rarity and it's usually a decent adult diaper (Abena L4 with plastic outer shell) as that gives me the assurance/confidence that I'm not going to leak for at least 4 hours. At night, when not feeling well and know I'll be asleep a minimum of 10-12 hours (frequently far in excess of these hours) then I use a Tranquility Super Plus contour booster pad (27 additional ounces absorbency) which fit perfectly inside of the Betterdry briefs/diapers I wear at night when I know I'll be asleep a minimum of 8 hours. IF I have to get up early for whatever reason I can get away with an Abena L4.
Booster pads come in all different shapes/sizes & especially QUALITY! I don't like the ones that bulk up a ton (these rely on a lot of SAP) but they also keep the wetness in contact with your skin, a bad combination especially when you're physically incapable of getting out of bed to tend to your own needs! I dislike the ones that have a tape strip to hold them in place as well and have never found another booster that works as well (especially on a cost per unit basis!) than the Tranquility booster pads with my almost exclusively relying on their contour booster pads, just do not unfold them as they'll render the standing leak guards in your main diaper useless and you will experience massive leaks!
Feel free to ask me anything on this as I have been living with this nearly 20 years now and have tried just about every brand/style in existence. What works for me may not be the best for you but at least I can give you input on what I have experience with.