There’s no “open vault from folder” on iOS. On iOS and iPadOS, Obsidian only has access to and vaults can only be in:
iCloud Drive/Obsidian
(iCloud)
and/or
On My iPhone/Obsidian
(local)
I haven’t used Logseq and don’t know how it works, but assume it only has access to its own folders on iOS as well (but could be wrong).