I'm creating this thread for @dfrost who had some questions in another thread about healing, but I didn't want to answer there, as I didn't want that thread to get off track.
So here's how Hospitals and healing work:
Your units naturally heal 1hp per game day if they are at rest inside a city, assuming that city has no Hospital. Each level of Hospital increases the number of hit points healed per day by one. It is important to note that this is rate is *per unit*. So, a stack of five infantry inside a city with a Level 2 Hospital heals 15 hp (for the entire stack) per day. AIrcraft heal the same way, you just have to have an Air Base in the city to get them there, and you have to leave the aircraft on the ground inside the city while they heal.
You mentioned that you have like 5 Level 1 hospitals in 5 cities. This used to be a popular strategy when hospitals use to improve morale, but they don't do that anymore. They do, however, increase population growth rate. So, if you want to use them to do that, that's fine. If we talking strictly about healing, though, you're usually better off to level a hospital or two up and get more healing benefit from fewer hospitals.
Field Hospitals (and Air Fields) can do the same thing to a lesser extent. You can only level them up to Level 3, and they don't get the benefit of the extra city healing point. But, you can put them anywhere and you can pair them with other field structures (combat outpost, airfield, etc.) to make yourself a nice little field position.
You can benefit from other people's hospitals if you have the right to have units in their cities.
Ships heal in coastal waters. I don't know what the actual rate is, but it's the best rate you're going to get for ships, so...
Here's a little personal Pro Tip with which others may or may not agree: I tend to build my (first) Level 5 Hospital in the (player) enemy capital I conquer. I try to manage my damage to that capital so that the enemy airfield remains intact after I have control of the city (an airfield needs to be at 40%+ in order to be operational). Once I've taken over the enemy capital and have an operational airfield there, I build my first Hospital there. Now, I have a Hospital to which I can fly troops and planes. Why build it there instead of my own capital or one of my own Homeland Cities? BECAUSE: That early in the game, I am usually very actively building other things in my Homeland cities. In this enemy capital, I already got an instant Air Base, and I can't produce units here until/unless I Annex this city later. So, just to restate, I build my first hospital in an early enemy capital because there's already an airbase, and so the Hospital construction doesn't get in the way of my other building construction in my Homeland cities.
So here's how Hospitals and healing work:
Your units naturally heal 1hp per game day if they are at rest inside a city, assuming that city has no Hospital. Each level of Hospital increases the number of hit points healed per day by one. It is important to note that this is rate is *per unit*. So, a stack of five infantry inside a city with a Level 2 Hospital heals 15 hp (for the entire stack) per day. AIrcraft heal the same way, you just have to have an Air Base in the city to get them there, and you have to leave the aircraft on the ground inside the city while they heal.
You mentioned that you have like 5 Level 1 hospitals in 5 cities. This used to be a popular strategy when hospitals use to improve morale, but they don't do that anymore. They do, however, increase population growth rate. So, if you want to use them to do that, that's fine. If we talking strictly about healing, though, you're usually better off to level a hospital or two up and get more healing benefit from fewer hospitals.
Field Hospitals (and Air Fields) can do the same thing to a lesser extent. You can only level them up to Level 3, and they don't get the benefit of the extra city healing point. But, you can put them anywhere and you can pair them with other field structures (combat outpost, airfield, etc.) to make yourself a nice little field position.
You can benefit from other people's hospitals if you have the right to have units in their cities.
Ships heal in coastal waters. I don't know what the actual rate is, but it's the best rate you're going to get for ships, so...
Here's a little personal Pro Tip with which others may or may not agree: I tend to build my (first) Level 5 Hospital in the (player) enemy capital I conquer. I try to manage my damage to that capital so that the enemy airfield remains intact after I have control of the city (an airfield needs to be at 40%+ in order to be operational). Once I've taken over the enemy capital and have an operational airfield there, I build my first Hospital there. Now, I have a Hospital to which I can fly troops and planes. Why build it there instead of my own capital or one of my own Homeland Cities? BECAUSE: That early in the game, I am usually very actively building other things in my Homeland cities. In this enemy capital, I already got an instant Air Base, and I can't produce units here until/unless I Annex this city later. So, just to restate, I build my first hospital in an early enemy capital because there's already an airbase, and so the Hospital construction doesn't get in the way of my other building construction in my Homeland cities.