Welcome back! Hope all is well now.
The plugin ecosystem has grown a lot. For linking and back-linking, I recommend the Note Link System plugin (mentioned elsewhere on this thread), and for a nice graph view you could use either this plugin or this one. In addition to the NLS plugin, an expanded panel that includes both back-links and forward-links is available via the Bundle plugin (with some additional features, but keep both plugins installed). One advantage of the latter plugin is that it also detects unlinked mentions (across your notebooks) of the current note based on its title. This feature may help you find where links are needed to be added.
So once your notes are imported from Obsidian to Joplin (you can import a whole directory of Markdown files, optionally with front matter metadata - tags, etc. - for each note, if Obsidian provides this), you may start linking them to one another with the help of these plugins. If this helps, you could use the Random Note plugin to sample from the pile of notes in no particular order.
I don't have a go-to video tutorial (perhaps someone else can recommend), but there are certainly many to choose from, and the techniques used on any app (be that Obsidian, Roam Research, etc.) apply for the most part here as well.