With the last battery operated device I worked on developing we reported the user 0% battery level when it was still at 15% actually, and we indicated 100% charge level when the battery was only at 85% (and the charger was also turned off at that level) - so we let the user charge and discharge anyway they want, and cheat a bit with the reported battery level, for protecting the battery itself, instead of relying on the user.

I imagine that most modern battery operated devices do something similar by default, unless they have a dedicated option to turn it on/off.

In order of priority:

1 - whatever they say in the manual

2 - if you'll be needing to use the device for a long period of time without the ability to recharge it, then charge it up to 100%

3 - otherwise, topping it up to 80% when it reaches 20% is a reasonable thing to do

It depends on the details of your application, battery chemistry and construction.

Typically avoiding the (dis)charge extremes is good. For Li-ion though a much wider range seems OK than (say) lead-acid.

This may help (I'm a paid user): https://apphousekitchen.com/feature-explanation-charge-limit...

The upper and lower percentages are the most stressful. Modern electronic lowers the voltage to charge them. So charging them before they reach it is a good strategy.

percentages are not as important as fast/slow charging

What do you mean? Fast charging depletes battery life?

I would say don’t worry about it and charge them to the full each time.

Electronics are consumable devices. When batteries degrade, you should replace the battery to give your device a couple extra years of life. I believe this is possible with most devices, especially phones and laptops.

Honestly, I just use freely and flip it when it starts performing poorly...

For lithium-ion batteries:

  "... keep your battery life somewhere between 40 percent and 80 percent ..."
  
  "Research shows that extremes wear out the lithium-ion batteries ..."

  https://www.eeworldonline.com/why-you-should-stop-fully-charging-your-smartphone-now/

Sony's recommendations:

  "Charge your device just as much as needed. If you leave your device charging
   for a long time after reaching 100%, the battery might be damaged."

  "Avoid total discharges, as these can damage the battery and decrease the
   capacity."

  "Limit exposing your device to extremely hot (37°C or above/direct sunlight)
   or cold environments (temperatures below 0°C)."

  "If you plan to store your device for an extended period, charge it between 
   20 - 80% and then turn it off (Tip: this applies to any device with a
   rechargeable battery)."

   https://www.sony.co.uk/electronics/support/mobile-phones-tablets-mobile-phones/xperia-1-ii/articles/00232835

   https://www.sony.com/electronics/support/articles/00255462