Thank you for your suggestions. Still, why is everyone pushing to a single nodeMCU instead of it being used as an adapter?
Because it gives you vastly more WiFi capability, is easier to code, avoids many interfacing problems, is cheaper and more compact.
I mean, how is people using ESP8266 with all the above I mentioned, for example? What do you do if you don’t have enough pins on the esp (nodemcu or other) and need to use an arduino mega / uno?
There should never be any need to use something as clumsy as an Arduino Mega or UNO.
You use I2C “port expanders” as explained by David, to interface other things such as keyboards or LCD displays. I2C requires only two pins, so you can build a whole system around an ESP-01(S) which has only four I/O available,
I have connected the TX of the Arduino to D1 and the RX of the Arduino to the D2, using a software serial on the ESP. No voltage divider. It’s working for the last 3 - 4 hours. Do you think that long term will cause some damage?
This abuse of the ESP is a longstanding point of heated discussion. Some suggest that the ESP can tolerate it, others not.
Is there any ESP8266 dev board that uses 5V?
No, apart from the input to the regulator, since the ESP is a 3.3 V device.