You definitely can, there are many ways to do so. Obsidian projects are basically just markdown files, so any file sync works.

Wrote a post of different ways you can sync a Obsidian vault/project: https://bryanhogan.com/blog/how-to-sync-obsidian

There are many ways to sync Obsidian for free. The official site lists several at https://help.obsidian.md/Getting+started/Sync+your+notes+acr..., or try https://github.com/denolehov/obsidian-git - and there are more.

iCloud works fine, and there are many alternatives as well, as mentioned in this thread.