I researched this myself a few years back. My limited understanding is that Joplin is the FOSS Obsidian alternative.
I install the Joplin flatpak and use One Drive as the sync backend without any trouble.
My take on Mark Down is itās essentially a tool to format text for people who never want to take their hands off the keyboard to format text as you often need to with a WYSIWYG editor.
Simplified BBcode is another way Iād describe MD too.
MD being plain text makes it very versatile too.
The history of it goes like thisā¦ there have , for many years, even predating wysiwig editors like Wordperfect, been markup languages. These are text with formatting commands embedded in the textā¦ eg like Tex and Latex.
The diference between a markup language and a wysiwig editor like Word or Wordperfect is that the source file is text , and the commands are visibly inserted in the text, whereas in Wordperfect the source file is binary.
So you can edit a markup language file with any text editor ā¦ like vi or nano.
Binary files are objected to because they can become proprietary ā¦ look at the number of times Word has changed its file format.
Markup is a proper Open Source language and file format.
Now, markdown is just a very small markup language. It has commands for headings, bold, italic, boxes and tables ā¦ that is about all. It is easy to remember the commands.
People use it to write simple notes and short articles ā¦ because they can keep the text (.md file) without worrying about any special non-open-source binary file type.
There are markdown editors which display the page of text alongside the rendered page. So markdown can be both text and wysiwig simultaneously.
Many forums, including ours , accept markdown formatting.
If I am writing a long reply, I do it in a markdown file, then copy/paste the file into the forum editor. While I am writing the file, I might use a markdown editor/viewer to check what it is going to look like, or if it is simple, I might just write it with vi.
I think you could only do āwysiwygā editing if your wordprocessor supported graphics modeā¦
I hardly ever used Wordperfect (I can remember doing desktop support and Iād go to work on someoneās DOS PC and theyād have the blue wordperfect screen on - like the ongoing joke about being unable to exit āviā - I had no idea how to exit it so Iād reboot the PCā¦
I did use the DOS version of MS Word (version 6?), and more so, IBM DisplayWrite 3 and 4 - both of those - if they werenāt in āgraphics modeā, you could highlight stuff and format it as Italic - but it wouldnāt display as Italicā¦ You could change the font, but paragraphs in different fonts, or different point sizes, showed the same anyway - it wasnāt wysiwyg, you couldnāt tell what it was going to look like till you printed itā¦ And that was excruciating on a dinky little Inkjet or even worse, dot matrix printerā¦
Dot Matrix from the show Reboot 
You could tell if Displaywrite was in āGraphics Modeā (I think you needed a VGA card - i.e. wouldnāt work on EGA or CGA - but I could be wrong) - your cursor would change from a āblockā to a graphical pointer like the cursor on Windowsā¦ Most apps by default, didnāt run in āgraphics modeā - I seem to remember Lotus 123 didnāt get āWYSIWYGā until version 4 (for DOS) - and WYSIWYG was part of their marketing - but - by then it was too late - MS Windows and MS Excel were dominatingā¦
Fantastic! It has a portable Windows version that runs on my Win11 AVD for one customer - so it should work on another VDI (Citrix then RDP to a Windows server ājumphostā)ā¦ Just tried it on MacOs - perfect!
Had a sandbox issue with the AppImage on Ubuntu - but was able to run it:
./Joplin-3.6.15.AppImage --no-sandbox
And the MarkDown document Iāve been working on displays fine on Windows, MacOs and Linuxā¦ Only issue is I canāt sync from within those Virtual Desktops (i.e. AVD and Citrix/RDP)ā¦ Theyāll only work with OneDrive and I mostly donāt have access to OneDrive from outside those environmentsā¦
I might try and get OneDrive working on Mondayā¦
I reckon my search for a MarkDown editor is over 
That is an important difference.
Even specialist markdown editors like remarkable are all keyboard affairs.
When Iām āworkingā (that includes posting stuff in this forum) I use a mix of physical traditional mouse device, and a trackpoint on my keyboard. e.g. I want to select a range of text in a terminal - Iāll use the mouse as itās more precise - then either mouse or trackpoint buttons to paste into a text editorā¦
Also - what I love about markdown is itās not āin your faceā unlike XML or JSON or HTML - itās still 100% human readable and editableā¦ it still looks like textā¦
Tried it - couldnāt wait - works perfectly fine for one customer (Joplin and OneDrive between Mac and Win11 AVD) - but another customer wonāt let me use it (OneDrive) in their VDI solution (Citrix). But Joplin (e.g. "Joplin.exe) portable works fine when I RDP to my main work area from Citrix. So my workflow there will probably be edit the markdown in Joplin, then email it to myself so I can get to it from āoutsideāā¦ The fact I can do this - makes a mockery of whatever security mindset dictated no OneDriveā¦ Itās bizarre : I can point my work supplied MacBook at their OneDrive - but canāt use the app or sync files inside the VDI (other than through a web browser - which also makes mockery of the feeble attempt to block OneDrive access).
Somebody started a github project for a cross platform, portable, onedrive client - but looks like they never made any progressā¦
https://github.com/gmdfalk/onedrive-portable/diffs/0?commit=6f7e07b635387d2655b2dabc19fcf23487ca1578&name=master&qualified_name=refs%2Fheads%2Fmaster&sha1=12c7eafe118f75b4db3050eb3dd5173dda3da5a8&sha2=6f7e07b635387d2655b2dabc19fcf23487ca1578&short_path=b335630&unchanged=expanded&w=false
Weāre also using some Atlassian stuff in that environment - itās been 10+ since I used Jira and Confluence - theyāre VASTLY superior and user friendly than the ugliness of other solutions (Iām pointing the finger at Service Now) - or kludgy 1990ās style āWikiā solutionsā¦ So I might see if I can use that stuff, maybe even just Confluence (which supports MarkDown out of the box) - instead of Joplin.exe - save the MD then email it to myselfā¦
Yeah, I like that too. Editing it in vi is no sweat.
Looks like you are settled on Joplin. Its great having choice.