There are 2 diff ways to keep a QT/Hospital tank.
One is the cycled method. I happen to use this only because I keep buying fish
LOL so I never have a chance to tear it down. In some ways its easier to have a cycled tank on hand; no major worries about ammonia/nitrItes, although you obviously have to keep an eye on it. One of the problems is, if you DO have to medicate with antibiotics, kiss that bacteria colony goodbye and you'll be left with an uncycled tank anyway.
Or, as some people do (and as I will once I stop buying fish
LOL) you can tear down and sterilize the tank between occupancies. Yes, its uncycled; it usually suggested to keep an extra filter sponge in the main tank to add to the QT tank if/when needed. Then you just pop the tank into the closet and its out of the way till you need it again. While its in use, keeping a very close eye on water parameters is necessary.
As for adding things to the QT, I barely add anything. It's bare bottom with a filter, a heater and some aeration. I will put in some plastic plants or a ceramic cave if the fish needs it (like clown loaches need a dark hiding space to feel safe and reduce stress), but other then that, not a thing. Less to try and sterilise perchance thats needed. I don't add anything preventative; so far that has worked well for me. For example, I've put 3 clown loaches thru QT and they have all come thru with flying colors, healthy and no sign if ich.
As for quarantine time, depends on who you ask. My fish get between 2 and 3 weeks in QT before they go in with the others; some recommend a minimum of a month and I've heard of others who won't add fish until they pass a 6 week quarantine. Keep in mind, if anyone winds up ill, EVERYONE in the tank starts QT all over again.
Hope that helps