is there somewhere a detailed description of the current Arduino IDE (1.8.13) ?
I currently use the UNO R3 as an ISP programmer for ATTiny's and I would like to know what the possible settings do/mean exactly.
For example tools-> Pinmapping, or
tools-> ATtiny25/45/85 (No bootloader)
groundFungus is correct. The Arduino IDE allows the authors of the "boards platforms" that add support for Arduino boards and bare microcontrollers to the Arduino IDE to define arbitrary "custom board options" menus. These allow the user to set configuration options specific to the board they currently have selected from the Arduino IDE's Tools > Board menu. The menus will only appear when the relevant board is selected.
These menus make the boards platform system extremely powerful and versatile. Even though Arduino hasn't made much use of the feature for their own official boards, some of the 3rd party boards platforms have used it extensively. I remember calculating that there are literally millions of possible configurations for the ESP8266 boards!
Since the boards platform developers can define any arbitrary menus they like, for any purpose you could possibly imagine, it is impossible for Arduino to provide documentation for these menus (other than documenting the "custom board options" framework itself). So it's up to the individual platform developers to document them. Probably the ones you're asking about are documented here:
I have another question about the ATtiny.
In a book it is recommended first to load a bootloader for a new ATtiny via Arduino IDE->tools->burn bootloader (at the bottom of the window).
But here no real bootloader will be loaded into the ATtiny, but only the "fuse bits" will be set. Is this correct ? Where can I find information about this in the net ?
It depends on which option you have selected from the Tools > Board menu. ATTinyCore provides board definitions that use the Optiboot bootloader to allow uploading over serial, ones that use the Micronucleus vUSB bootloader, and ones that require using a programmer to upload. There is some information about that here:
There is a ton of information (maybe not organized as well as it could be) in the documentation for the core. I believe the bit about burn bootloader on non-bootloader configurations is in that list of quick gotcha's at the top of the readme. It is an eternal point of confusion.
I wish they let the core override the text for that option based on the selected board, so if a non-bootloader board was selected, we could change it to say "Set fuses"