If you want to use SPI-
The homepage of the library you mentioned, clearly explains the circuit connections.-
The MPU-9250 pins should be connected as:
VDD: this should be a 2.4V to 3.6V power source.
VDDI: digital I/O supply voltage. This should be between 1.71V and
FSYNC: not used, should be grounded.
INT: (optional) used for the ...
The firmware update over WiFi is called OTA update. OTA stands for "over the air".
The esp8266 can OTA update itself. The Arduino library for this is called ArduinoOTA and it is bundled with the ESP8266 Arduino boards support package.
See the example for ArduinoOTA library in IDE Examples menu.
Other MCU have the self update option too. ESP32 has ...
To better understand voltage divider, one can look at this online calculator https://ohmslawcalculator.com/voltage-divider-calculator to see the actual formula and try the calculator to compute the unknown wanted.
It is not well publish the existing voltage divider on the NodeMCU, of course you need to browse the schematic to figure it out.
It goes the same ...