You can match most straps with any Apple Watch Series 3 or later case of the same size.
38mm, 40mm, 41mm and 42mm straps are compatible with 38mm, 40mm, 41mm and 42mm case sizes. 44mm, 45mm, 46mm and 49mm straps are compatible with 44mm, 45mm, 46mm and 49mm case sizes.
The Solo Loop and Braided Solo Loop straps are only compatible with Apple Watch SE, Apple Watch Series 4 or later and Apple Watch Ultra or later.
For Solo Loop and Braided Solo Loop, the 40mm, 41mm and 42mm cases are compatible with strap sizes 0–9; the 44mm, 45mm, 46mm and 49mm cases are compatible with strap sizes 0–12.