What is the right way to charge my devices?

percentages are not as important as fast/slow charging

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

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

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...

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.

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.

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