Fleet-carrier ships

Contrary to what you may think, the name doesn't mean the ships carry smaller ships in battle, like fighters. They do carry ships, but only other warships, as fighters are essentially extinct, at least when it comes to space combat.
More of a role, than a proper ship type, though dedicated ones aren't uncommon.
The ability to travel faster than light via FTL travel is only really viable on a larger scale, one smaller military vessels really can't afford. The smallest a ship can go while having a powerful drive would be something like the Star-Reaver, which had to have important functions stripped to allow that, like Shields or a life support system. Something like the Stalwart Sentinel really can't do that. It's just not worth it as an investment. They need to allocate that power to other things, like weapons or shields.
The problem existed even before FTL, as smaller ships had neither the fuel nor the thrust to get to relativistic speeds necessary for interstellar travel in a reasonable timeframe.
But these smaller vessels are still quite valuable in combat. So, a method was devised.
Just stick the smaller ships on a bigger ship capable of interstellar travel.

Prevalence

A standard for mobile, large fleets. These ships are very common in any military, as those smaller ships are used quite a bit. Though, the distinction between any large warship and a dedicated fleet-carrier is barely there. Any large ship can have smaller ships attached to it, and a smaller ship can attach to any big ship.

Methods

The basic gist of it is simple. Strap a small ship with no interstellar travel capacity onto a ship with interstellar travel capacity. It's even remarkably similar between FTL and sublight travel. The major difference there is that with sublight, you get a bigger shield of ablative ice in the front, to cover all of the attached ships. In FTL, the warp bubble is expanded to cover those vessels, at the cost of a bit more energy.
When the ships reach the target destinations, the smaller ships detach and begin operating as usual. No combat deployments here, STL (slower-than-light) transit takes years to slow down and FTL navigation is imprecise enough to have a hard time dropping in the middle of combat. Also, if a bigger ships with antimatter thrusters or reactors gets hit properly, those ships attached are not going to survive.

Variations

Practices vary between militaries. In smaller ones where each ship is invaluable, they commonly use proper warships as carriers, which works well enough. Really any ship with interstellar capabilities will do, battleships and above with STL, and cruisers and above with FTL, by common classification.
In bigger, more established militaries and fleets, there exist dedicated fleet-carrier ships. These ships are very long, the smallest being upwards of ten kilometers long. They're lightly armed and armoured, with a lot of space dedicated to fuel to ensure maximum fuel efficiency. The bigger of these ships can hold tens of even cruiser-size ships, which are a few kilometers long in general.

"Alright, crew of the Praesidium. Preparing interstellar transit, you know the drill. Strap yourselves to something, or preferably go into stasis, cryo or preschamber, and get comfy. We're going to be sticking with the Dubiously Vainglorious Ascent for a change, and acceleration at 2.5g will begin in T minus thirty minutes. You know by now how that feels if you're not strapped in, so I recommend getting on with it."

— Fee Las, 2621