How to concatenate Sass variables

The best to do it via interpolation ( #{} ), where all strings become unquoted.

$basePath= 'example.com';

#some-div{
    background-image: url('#{$basePath}/images/photo.jpg');
}

This is a safer way for this particular use-case as you won't want any additional quotes added to your url.

A example:

$dot: '0.'; 

@for $i from 1 through 9 {
  .op#{$i} {
    opacity: #{$dot}#{$i};
  }
}

By logic, the variable is declared in $dot: '0.'; and I called her in #{$dot}. This example above shows two concatenated variables in SCSS.

Use this:

$domain: 'domain.ru';
#some-div {
    background-image: url('#{$domain}/images/img.jpg');
}

Multiple issues here, the correct syntax is

$url: 'siteurl.com';

#some-div{
  background-image: url($url + '/images/img.jpg');
}

• When you assign a value to the variable, you need to use : and not =
• You should quote the image path as it's a string.
• Remove the stray + symbol before $url

You can see the above code in action at SassMeister