The best open source alternative to Logseq is Joplin. It's not free, so if you're looking for a free alternative, you could try Standard Notes or Simplenote. If that doesn't suit you, our users have ranked more than 50 alternatives to Logseq and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Logseq are Zettlr (Free, Open Source), Trilium Notes (Free, Open Source), QOwnNotes (Free, Open Source) and Vnote (Free, Open Source).
Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, can be copied, tagged and modified either from the applications directly or from your own text editor.
Zettlr is a supercharged markdown editor that combines many writing editor features and collect them in one application. It aims at speeding up your workflow and help you write without any interruptions.
The problem with notetaking apps like Evernote or Apple Notes is that once you write a note, after a week you never look at it again. Without any organization, you're stuck praying that search will work for your future self.