I am running my aluminum service entrance cable from the main panel to a sub panel in my garage. It runs from the garage under the house then out the wall, makes a 90 degree turn to go into my side of the main box. I cannot find the code that says what kind of conduit it should be in when making the transition to outside and into the box. Any suggestions?
How deep do you go? You can give metal Rigid (/RMC) or IMC conduit only 6" of cover. Other conduit needs 18" of cover. If you want to direct bury, you need 24" of cover.
Since you are downstream of a main panel that presumably serves other loads, this is not service wire, this is feeder, and needs to be sized as feeder not service conductors. You need to run separate neutral and ground wires, unless you are running in IMC or Rigid conduit, in which case you are allowed to use the conduit shell as the ground wire.
Don't run cable in conduit, that is the mark of the amateur. It is also a very, very hard job. Either direct bury at 24" or use THWN-2 wires instead of cable.
It can be conduit or direct burial. Above ground to 18" below ground it would need to be schedule 80 PVC or could be rigid metallic or IMC. Below 18" schedule 40 would work. If you go with direct burial it needs to be 24" with the sections above 24" protected by schedule 80 or rigid metallic or IMC as a quick overview. Table 300.5 in the NEC.