drain pipe depth under concrete
You comment - "Sorry, that's really a cra**y answer. " is not a constructive answer. - A moderator is not supposed to be an expert. A moderator volunteers his time to make sure the questions are proper and answers are to the point and to eliminate the "trash talk" or other off-the-point comments.
You have no basement and your connection elevations are really not that important to anybody but you. Exterior sewer main connections are beyond the homeowners control and are what they are.
By "drain pipes" that term is more closely applied to water drainage than sewerage. "Sewer pipes" usually refer to sewerage and gray water disposal.
To try to answer the poster's problem constructively -
If the question refers to any drain tile installed to dewater the area, if possible, the bottom of the drain tile should be level and slightly below the bottom of the footings. This applies to perimeter drains and to general dewatering drain lines. If not possible, the should be as deep as possible. The deeper, the better they work to reduce the water and pressure under the slab. Drain tile does not have to be sloped. In some areas, water from drain tile is not permitted to flow into the sewer, while in other areas it is permitted. The slope of the connection would be as necessary to connect.
If the question refers to sanitary sewer connections, I assume the poster knows/found the location of the sewer pipe to the ouside main. If possible, the slopes should be as uniform as possible and any changes in slope between the interior sewer pipes should be minimized by matching the interior slopes with the exiting slope. Even if there is no permit required, the municipality can provide information on the approximate slope of the exit line. If you plan to lower the elevation of the lines to the main, elevations can be obtained. Changes in slope can be as bad as a blockage or a slope that is too steep, which causes separation of the matter.