Battery Cycle Count

Heat is possible, mainly on iPhone, iPad if the device is in heavy use during charging, where the drainage is pretty much equal to the amount of charge the device is able to get through a given charger. Although generally, you shouldn't be concerned about that. If it gets really hot (too hot to handle) you should put it down and give it some rest, as a general piece of advise. Things like that can also be a bug in software. I've seen some people report such a thing.

If your phone doesn't drain significantly over night and you're still still getting through the day with, I don't know, 70% percent battery left in the evening, there is no need for charging. Although you could. It's up to you. Generally it is not advised to leave those type of batteries (Lithium-Ion) charging all the time, because it degrades the battery internals if it is kept on 100% all the time. Even worse is letting it drain down all the time. In case of a MacBook, it is programmed that if it is plugged in, the battery is being charged but the device is powered by the current coming from the power socket. When the battery is charged fully, charging is stopped and only tops off the battery every once in a while to stay full. But keeping it plugged in whenever you can while you are using it basically is the way to go.

These are mainly mobile devices so, they are designed to be used on battery power, especially iPhones and iPads. MacBooks are somewhere in between. You shouldn't let the charging thing be an inconvenience to you though.

Simple practice: If you are/going someplace, and you have your charger with you and you know you are going to be staying at that place for like at least an hour, and have an opportunity to plug it in, why not plug it on instead of draining the device!

My Macbook is sitting on my desk pretty much all day, and is plugged in. I unplug it at night when I shut the lid and go to bed.

Cycles counts essentially show you how often the battery has been drained and recharged fully. Ergo, the battery is cycled.

Being on 100% and draining it down below 20% or so and then charging back up is considered 1 cycle.

Being on 100% and draining it to 90% and plugging it back in is only a fraction of a cycle. You would have to do this exact thing 9 times to complete one charge cycle.

So in short and as an example: Leaving your Mac plugged in, unplugging at night, and plugging it back in when you are using it could give you one cycle every week or so. If one day, you take your Mac with you, and drain the battery, you will have put one cycle on the battery in just one day. If you did recharge in between, you could even put two cycles on it. And depending on the general health of it, even more because the worse the battery the more often you have to charge, resulting in ever worsening of the condition. That's why it is important to keep a battery healthy early on and for as long as possible. Putting cycles on a battery is inevitable since it is a mobile device, however the user can do their fair share of not putting more stress and usage on it than necessary.

https://www.apple.com/batteries/maximizing-performance/

https://www.apple.com/batteries/why-lithium-ion/