I've never seen stranded wire in an ethernet cable, but I see it's a thing. I occasionally turn my male-male ethernet into male-female with a keystone block and never see stranded.

I dabble in electronics and I eliminate stranded wire whenever I find it. Some people love it but I find it really inconvenient.

Solid Ethernet cabling should generally only be used on permanent building cable that never move. Stranded should be used on patch cables and cables that get moved around. Beyond being more difficult to handle, regular movement will break solid copper, often leading to intermittent connectivity.

Stranded ones are very common in patch cables.