I am doing research on best practices for my lithium batteries and lifepo4 powerstation. There's some conflicting opinions and variation for cycle numbers.
Will leaving my things plugged in at 100% hurt it more than constantly unplugging at 80% and replugging at 20%?
My understanding is that it depends on the device, and most modern devices take care of what's best by themselves
That's not true.
Lithium batteries have a longer useful life if not allowed to drop too low or charged too high. 20% and 80% are typical values. Ideally they would be at 50% SOC and that's why most batteries in new devices will arrive charged to around 3.6-3.8v.
This creates a problem for device manufacturers because if they force the device to treat the battery well, users won't get as long between charges. They will sometimes give you options (most laptops will have a setting to stop it charging beyond 60% or 80%, some phones will have a setting to stop it charging to full) but they'll advertise the full battery runtime they can squeeze out while damaging the battery and that will be the default setting.
Convenience dictates that you may need to charge above, or discharge below, the recommended levels. Which would be much less of an issue if batteries were easily replaceable. But increasingly, they're not.
tldr; manufacturers have zero incentive to make sure their devices treat the batteries well
as long as they survive the warranty period
iPhones figure out when you usually take your phone off the charger (e.g. based on your wake up alarm setting, but I think it also does some machine learning) and when you charge your phone it only charges it to 80%. Then, just before you are expected to take it off the charger in the morning it charges it to full. That way the battery spends less time above 80% charged.
My android pixel does the same.
They don't outside of not doing things that cause acute damage to the battery. They can't because "best" is situational.
If I have a phone with an empty battery and I'm going out all day starting in an hour, best is to charge as fast as possible to 100%. That's the most wear I could put on the battery out of any charge cycle, but going easy on the battery isn't my first priority in that scenario.
On the other hand, if I have all night to charge and won't be away from charging for more than a few hours the next day, best is to spend most of the night charging to 60% and stop there. It's an order of magnitude less wear than the above, maybe more.
For best service life, avoid fully discharging[1] the battery, charging it above 60%, storing it long-term charged over 60%, getting it hot, or charging it in less than several hours. In most devices, you don't have the ability to control any of that so the best you can do is plug it in at 20% and unplug it at 60% (or 80% if you need the extra runtime - it's still better than 100%). I'd like to see consumer devices get an "eco mode" or some such to select battery-preserving behavior manually, but that's not in the interests of device manufacturers who want you to buy something new when the battery wears out.
[1] An actual full discharge to zero volts causes acute damage to Li-ion batteries and most devices won't let you do it