Where the files go in DT - See Preferences > Import. There’s an options button under the heading New Notes. Click on it.
The most interesting option is Automatically Classify. DT/DT Pro will attempt to place the new content in the group(s) that it finds most appropriate, based on a comparison of the text of the new content to the text characteristics of documents in existing groups. This feature works best when you have already organized a substantial number of items by group.
Another option is to import new content to the top-most or current (frontmost) group.
Another option (that I commonly use) is to designate a group to receive new content. My target group is named Incoming. Note that even with this setting some operations will send new content to the topmost group.
 Where files go in DEVONagent - When you run a search set the results are temporarily held in cache.
You can save selected or all search results to the DA Archive, which will store the files on your hard drive.
And/or you can transfer selected files to your DT/DT Pro database, which will store the files in the database files on your hard drive.
Note that your DA cache may grow very large. In DA preferences there’s an option to clear the cache on quit. There’s also an option DEVONagent > Empty Cache to allow manual clearing of the cache at any time.
 Browsing uses hard disk space? Not really any more than other browsers. Of course, if you have set up a number of Tabs to open pages this can require considerable memory, and if you run out of physical RAM that memory will be provided via Virtual Memory swap files on your drive. And of course the same applies to DA cache files.
The Archive is located in your Home directory at ~/Library/Application Support/DEVONagent/. Tip: Close DEVONagent. In the Finder, select (don’t open) the Archive folder. In the Finder toolbar, click on the Actions button and select the option Create Archive of “Archive”. A compressed copy of the Archive will be made and you can move it to any location of your choice, e.g. in your Documents directory. If you ever need to use the backup, double-click it to unzip it, then place it in the proper location inside ~/Library/Application Support/DEVONagent/.
Backing up DT/DT Pro - Whenever I’ve made a substantial investment of time and effort in adding new content or writing/editing, at break time I will run Tools > Verify & Repair followed by Backup & Optimize. Now I have a recent backup and don’t have to wait for an automatic backup.
The DT Personal database is a folder named DEVONthink. An archive backup can be made in the same way as for the DA Archive. The DEVONthink folder is located at ~/Library/Application Support/DEVONthink/. Always close DT before doing this!
External backup archives of DT Pro databases can be conveniently created using DT Pro Scripts > Export > Backup Archive. Backup Archive creates the smallest possible compressed and dated archive of the open database.