Rental Asset Expense Catagory in TT

How would the reconstruction of a rental garage be catagorized?

Under MACRS, that's classified as residential rental real estate, if the garage is in fact, rental property or a part of a rental property that you lease out.

Where can I find the detailed descriptions for these catagories?

There really is no "detailed" description, as it would be absolutely impossible to cover every single little itty bitty possibility. But rental property is covered in IRS Publication 946 at https://www.irs.gov/pub/irs-pdf/p946.pdf and specifically addressed on page 31 of that document.

For the closest thing you'll see to a "detailed" description of anything when it comes to depreciated assets, see Appendix B of that document.

For your garage, in the COST box you'll enter what your total cost were for the newly built garage. Then you enter ZERO in the Cost of Land box since your reconstruction did not involve the acquisition of any additional land.