Advice for Multi-site Setup

Hi @prevoj and welcome!

I've read the files in the sites folder but they don't really provide much info. 

Hm, it sounds like this is something we should improve.

I don't know how similar or different the multi-site processing is between D7 and Backdrop.

In general, not much has changed. The one (maybe only) change is that the "magic" domain name matching has been removed. It's now *required* that you list your site's domain name -- and the folder its files will live in -- in the sites.php file.

Here's a copy of one my sites.php files:

$sites['examplesite.com'] = 'examplesite1';
$sites['examplesite.biz']   = 'examplesite2';

Once you've done that, it should be business as usual if you are familiar with Drupal 7's multi-site set up!

I usually put site-specific modules, themes, layouts, and files in the directories specified in sites.php, but I *also* move my configuration files outside my web root (instead of leaving them inside the files directory).

For my sites, I also change the location of my config files in each sub-sites settings.php file, so it looks something like this for examplesite1/examplesite.com:

$config_directories['active'] = '/var/www/examplesite.com/config/examplesite1/live-active';
$config_directories['staging'] = '/var/www/examplesite.com/config/examplesite1/staging';

... and like this for examplesite2/examplesize.biz:

$config_directories['active'] = '/var/www/examplesite.com/config/examplesite2/live-active';
$config_directories['staging'] = '/var/www/examplesite.com/config/examplesite2/staging';

These config changes are optional, but I find that I like to have all my config organized just like my sub-sites directories, for sanity :)

I just found this discussion in the Backdrop CMS issue queue regarding multi-site. If you are interested in or using Backdrop CMS with multi-site, I recommend you read this and/or provide comments of your own. 

https://github.com/backdrop/backdrop-issues/issues/3206