It does look like calendula.

Seeds, BTW, can spread far and wide by a number of different means, one of which is through animals &/or birds either ingesting and eliminating, or picking up and dropping. Sometimes seeds stick to an animal's body/fur/feathers and become dislodged later. Seeds can also be carried far and wide by water and wind.

Even us humans can move seeds around if they stick to our shoes or clothing.

It's growing in an excessively-drained gravel/rock pile, so certainly doesn't need good soil. Gave it a good drink to reward its persistence. Didn't catch its flower, but it looked just like this.

If you keep deadheading it (removing the dead flowers) so that it cannot set seed, it will keep blooming more and more prolifically. If you allow it to set seed you can either collect the seeds and plant them intentionally next year, or let it self seed around the area. Calendula are very easy to grow from seed or to allow to naturalize and self-seed year after year.