  Topological insulators have attracted great interest as generators of
spin-orbit torques (SOTs) in spintronic devices.
Bi\textsubscript{1-x}Sb\textsubscript{x} is a prominent topological insulator
that has a high charge-to-spin conversion efficiency. However, the origin and
magnitude of the SOTs induced by current-injection in
Bi\textsubscript{1-x}Sb\textsubscript{x} remain controversial. Here we report
the investigation of the SOTs and spin Hall magnetoresistance resulting from
charge-to-spin conversion in twin-free epitaxial layers of
Bi\textsubscript{0.9}Sb\textsubscript{0.1}(0001) coupled to FeCo, and compare
it with that of amorphous Bi\textsubscript{0.9}Sb\textsubscript{0.1}. We find a
large charge-to-spin conversion efficiency of 1 in the first case and less than
0.1 in the second, confirming crystalline
Bi\textsubscript{0.9}Sb\textsubscript{0.1} as a strong spin injector material.
The SOTs and spin Hall magnetoresistance are independent of the direction of
the electric current, indicating that charge-to-spin conversion in
single-crystal Bi\textsubscript{0.9}Sb\textsubscript{0.1}(0001) is isotropic
despite the strong anisotropy of the topological surface states. Further, we
find that the damping-like SOT has a non-monotonic temperature dependence with
a minimum at 20~K. By correlating the SOT with resistivity and weak
antilocalization measurements, we conclude that charge-spin conversion occurs
via thermally-excited holes from the bulk states above 20~K, and conduction
through the isotropic surface states with increasing spin polarization due to
decreasing electron-electron scattering below 20~K.
