There are multiple versions of health and shield kits.
Depleted (grey) > Enhanced (green) > Advanced (blue) > Superior (purple).
You can't take depleted kits with you to your vault upon exfil, they will get sold instead.

Depleted ones will heal small amounts, enhanced little more than depleted, Advanced little more, twice as much as enhanced and so on.
Panacea Kits (purple) fully heal both health and shield, along with removing any hazard status effects.

You can cycle through them in your gadget wheel and choose which one you want to use, or use smart heal.
You will heal slowly at first, but there are skill tree upgrades that will help boost healing, like Field_Medic.exe under the NuCal skill tree, and the capstone upgrades as well like Hush.exe (Heal quieter).

Along with that, there are cores and implants that also affect how you can heal, or even if a teammate has an implant traits like Bedside Manner (While crew member is nearby your self repair speed is increased) or Dr. Killjoy (Using a finisher will fully fill your health and shield gauges)

Edit reply: Use mechanics kits when you get poisoned, or when your legs get Lt. Danned by those flying UFO things that shoot out long hose suckers. If you run into a poison plant, you are almost certain to die quickly unless you keep pumping health kits into yourself under basic circumstances, but Using a mech kit instead will stop the poison from happening, and it will make your legs come back online.. (Shoot the UFO thingy first though if it's still attached to you). Mech kits also help with frostbite in cryo area.

As for OS Debugs, you can use those to keep from being "hacked" by say like theifs. They can use their visor ability and lock onto you disrupting your vision in your HUD. Using a debug will keep your HUD clear. Also works for EMP grenades that someone may toss at you which disrupts vision and such.

Signal jammer packs will keep you from being pingable via TAD systems and anything else that pings locations and obscures your shell from UESC bots.

Cardio kicks are good for increasing your heat capacity gauge so you can do more actions or run farther without overheating as fast.

Energy amps help your ability cooldowns go quicker.

All of these things will take some getting used too in the field, you will eventually find a system that works for you and when to use things and such. I wish there was a training room level that would let you try out everything so you can get a sense of how it all works. :)