How do I open from the same folder in the open dialog?

Go to Tools | Customize… and go to the File page. Check Remember Last Folder.

You can alternatively specify a folder to open in with the EmEditor shortcut. In File Explorer, open Properties for the EmEditor shortcut, and add /cd in the Target text box. (Enabling /cd will set the current directory as the default folder.) For instance, if the install folder of EmEditor is C:\Program Files\EmEditor, enter "C:\Program Files\EmEditor\emeditor.exe" /cd. Next, change the Start in field with the folder you want to open from.