Sloeber installs Sparkfun esp8266 version 2.1.2 in the folder esp8266/hardware/esp8266/2.1.2 (different from arduino IDE)

[jantje]

According to the arduino documentation https://arduino.github.io/arduino-cli/1.1/package_index_json-specification/

name: the folder used for the installed cores. The vendor folder name of the installed package is determined by this field

This means the package name is the folder under packages which in this case is "SparkFun".
Therefore Sloeber installs in the wrong folder.

[jantje]

Investigating a bit more in debth it seems the sparfun provided json file contains 2 packages sections and the one containing esp8266 platform is named esp8266.
Arduino ide documentation states

3rd party vendors should use a single PACKAGE_XXXX ...

So my cureent understanding is that

1. Sparkun provides a json file not complient with Arduino guidelines.
2. Arduino IDE does not use the name of the package as documented.

[jantje]

arduino/arduino-cli#2759

[jantje]

Given that there seems to be consensus that Sloeber install in the right place I'm closing this one.