You can match most straps with any Apple Watch Series 3 or newer case of the same size. The 41mm straps work with 38mm and 40mm cases; the 45mm straps work with 42mm, 44mm, and 49mm cases.
The Solo Loop and Braided Solo Loop straps are only compatible with Apple Watch SE and Apple Watch Series 4 or newer. The 40mm and 41mm cases work with strap sizes 1–9; the 44mm and 45mm cases work with strap sizes 1–12.