Items and magic can be used to heal at any time outside of combat, while Blue Benches and Inns fully restore your team.
You may need a full ATB bar to use an item or heal in combat, but you are free to do so anytime between battles. You can do this by clicking on X to bring up the command menu, and you can cycle through the characters with the triggers.
Just select what you want to do, choose your target and you're good to go! you will heal them (this is also useful for purging negative states such as Poison). If you want to initiate multiple heals or use more than one potion, you can do so by holding down the L1 key to enable multicast (which makes the process a lot less tedious).
One thing to note is that you cannot do this from the main menu. In classic Final Fantasy titles, simply open the inventory, select the potion you want to use, and then the character (s) that need a supplement. Oddly enough, this doesn't work in Final Fantasy VII Remake, nor does the use of magic in the Magic menu. If you want to heal between fights, you have to do it in the field.
In addition, you will find blue benches next to vending machines dotted around dangerous areas. Vending machines are a great way to stock up on critical potions, ethers, and phoenix down, but the bench is easily the most important item here. Selecting it with Triangle will allow you to rest and restore all HP and MP, much like a hostel.
Speaking of hostels: they are rarer than benches, but work the same way. Cloud's apartment in Sector 7 slums offers a free bed to rest your tired head, but you'll soon notice that beds and hostels are more meant for healing between major story beats. Most of your rest will be on benches, pure and simple.
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