The menus in Discourse are … not great.
On a PC, when logged in, click your profile picture at top right.
Then select the picture at the bottom of the vertical menu that appears, with a tooltip “Profile”.
Then “Preferences” on the horizon menu,
and below that “Email”.
Then scroll down and tick “Enable mailing list mode”.
This gets forum posts and PMs etc. mailed to you. You can reply by just replying, and your reply will appear with an “email” icon in the topic. Threading is preserved in email messages even though it isn’t on the site(!). See also here. I use filtering at the email server to move some Discourse mails to a subfolder to avoid getting overwhelmed.