When I started to develop plugins for Notepad++ the following links have been helpful:

How to develop a plugin or a lexer
Messages and notifications
Sources of plugin interface
Scintilla documentation

Regarding toolbar buttons

The handling of toolbar buttons is totally transparent to a plugin developer because they work like a menu entry.

A plugin developer has to fill an array at plugin’s startup that contains structs of type FuncItem. Every array element describes a menu entry of the plugin (text, click handler function, menu command id, Checked state, and keyboard shortcut).

Notepad++ queries a pointer to this array at a certain time in the startup process. It generates the menu entry of the plugin in the Plugins menu, adds for every array element a submenu entry and fills the menu command id member of the array elements. So you know the menu command ids of your menu entries.

If you want to have a toolbar button you have to hook the NPPN_TB_MODIFICATION event sent by Notepad++ to your plugin. In the handler of this event you have to fill a struct of type toolbarIcons (please note: you have to fill only one of its two members, either hToolbarBmp or hToolbarIcon but it seems only the first one works). The bitmap whose handle you provide has to be in a 8 bits-per-pixel palletized format (max. 256 colors) and its size has to be 16x16 pixels. Then you can send a NPPM_ADDTOOLBARICON event to Notepad++ where you provide the menu command id of the menu entry which you want to connect with your toolbar button and a pointer to the toolbarIcons struct you filled before.

You can write plugins in a lot of languages. Notepad++ itself is as you said, “classic Windows code”, C++ and straight Windows API function calls, so maybe the natural choice for a plugin is to stick with same.

the code here is classic Windows code without even MFC or ATL/WTL I assume?

Yes…double-edged sword I guess. Personally, I’m glad it is not MFC-based. I always found MFC to be a joke.

of course i did not mean you, but big, big enterprises.
i hope this was a joke from you, because otherwise maybe you need a better translator after all 😉

yes, and most of the plugins are open source.

but there are many developers that published source code, and had a very bad experience, as some rogue entities took profit on selling it, and worst case, trying or accomplishing to patent, what they have taken from another person’s open source project.

so i can fully understand why some developers, especially the ones with a lot of bad experiences, choose to omit to publish any further source codes.

And the authors of these plugins helped me with advice on how to compile these projects! For which they thank you!

every free plugin developer has the right to freely share his source code for free, as well as having the free right to omit any further free publication, to eventually protect his and/or her intellectual property.

additionally: as soon as you would be able and ready to modify, for example @dave-user 's customize toolbar plugin, you will have enough knowledge to write it from scratch, as you will be quicker doing that than to read and comprehend the existing code.