Can Apple Watch SE 40mm bands fit a Apple Watch Series 11 42mm?

Short answer: No — your Apple Watch SE 40 mm bands will not fit a 42 mm Apple Watch. Apple uses two band-size standards that must match the case group.

Apple’s band compatibility is based on case size families, not specific model numbers. Bands for 38 mm, 40 mm, or 41 mm watches all fit each other’s cases. Likewise, bands for 42 mm, 44 mm, 45 mm, or larger watches all fit with each other. But the smaller family (40 mm) is not compatible with the larger family (42 mm) because the slide-in connectors are physically different.

on each band on their site they list compatible watches for sizes

Works with most versions of Apple Watch.

See compatibility details