The minimum depth of a sewer line depends on the location.
However you must put at least the part of the sewer line coming out of your house into the ground at least 1 foot or 15 inches below ground.
Then as the sewer line leaves the house it must start to go deeper and deeper until it reaches the main sewer line.
You want the sewer line from your house to be at a slant so that the sewage runs out of the sewer line properly and doesn't get stuck in the sewer line.
Sewer lines at the alley or street can be as deep as 7 feet or even as deep as 15 feet and sometimes deeper depending on where you're located.
You'll need the city to dig up the sewer line in the street to connect your sewer line too.
And then you or a plumber digs up your sewer line and digs until you reach where the sewer main is and then connect your sewer pipes to the cities sewer pipes.