A yorishiro can be almost anything, a tree marked with a shimenawa rope to show it has been set apart, a rock, a pillar erected specifically for a festival such as the Onbashira pillars raised at Suwa, or an object carried in procession, and in most cases nothing about its physical construction is special, a tree is not carved or altered, what matters is the marking that designates it.
How something becomes a yorishiro
Shrine priests designate a given tree, rock or pillar through ritual, typically the same rope tying ceremony that produces a shimenawa, and it is that act of marking, not any manufacture, that turns an ordinary natural object into a vessel a kami may temporarily occupy, a rock beside it with no rope remains simply a rock.
Why this is not the same as a shintai
A yorishiro and a shintai are frequently confused, but the distinction is real, a yorishiro is a temporary dwelling, used for a single festival or a season and then released from that role, while a shintai is a permanent, singular vessel installed once and never replaced. Confusing the two misrepresents both, treating a seasonal marker as though it carried the same unbroken, generations long continuity a shintai is specifically defined by.