will straps for Apple Watch series 9 41mm fit apple watch series 11 42mm?

No — a strap made for 41 mm (i.e. the “smaller-case” size) will not properly fit a 42 mm Apple Watch (the “larger-case” size). According to Apple, bands for 38/40/41 mm cases only fit those same small-size watches, and bands for 42/44/45 mm cases only fit the larger-size group.

Ref: Remove, change, and fasten Apple Watch bands - Apple Support