[java-man]

They say it's XChaCha20-Poly1305 [0], and "no one but you" can read your private notes [1] (I don't know what that means).

They also list some security audits, though not without problems.

[0] https://standardnotes.org/help/3/how-does-standard-notes-sec...

[1] https://standardnotes.org/help/1/who-can-read-my-private-not...