Installing a new sub panel in my detached garage, about 90' from the main panel. Half of the distance of the run will be under the house and the second half underground to the garage. I have read that SER should not be run in underground conduit. Should I use a combination of SER and URD cable?
Run conduit the whole way and use individual XHHW-2 wires
First off, if you're going to run conduit, simply run the entire run in conduit -- it's not all that much more expensive than a cable wiring method parts-wise, and running a fat (say 2") Schedule 80 PVC conduit for the whole run means you can put fatter wires in in the future if called for, as well as making the pull relatively easy for whoever (you or your electrician) does it.
As to what goes in the conduit, once you have a conduit for the whole length of the run, you can run individual wires (presumably XHHW-2 insulated AA-8000 aluminum alloy wires at the sizes you'd be dealing with for 100A) for the hots and neutral, and a relatively thin (8AWG) bare copper ground. This is no more expensive than either a SER or a URD cable in the grand scheme of things, takes up less space in the conduit, and is far easier to pull through the conduit than a fat, stiff cable will be.
I agree with @ThreePhaseEel. In fact it would probably be cheaper and less laborious. Now there is nothing in the NEC that says you can't run SER or URD in conduit underground that I know of, but you cannot use it as direct burial cable.
That being said, if you use @threephase recommendation you would be able to run a minimum #3 XHHW-2 copper and ground in a 1 1/4 pvc conduit. I would go with a 1 1/2 for an easier pull. If you used SER copper in order for you to meet the 40% fill qualification you would have to go with a 2" pvc or larger. Not to mention pulling it would be a bear. You can just trust me on this and try and keep your bends to a minimum no more than 360 degrees without a pull section.
If you are trying to save money you could go direct burial aluminum USE-2 cable, minimum size conductors #1.