“How can they change anyway?”

All DNA in every cell is subject to “random” mutations over time as cells replicate. DNA damage from radiation and/or oxidative stress causes the changes. Very rarely, there is an accepted mismatch in replication.

To your other point. Horizontal gene transfer is not a whole genome switch, but a rare event of a transposable element arising in a different species. Horizontal transfer of transposable elements is poorly understood and the underlying mechanisms are still largely unknown. You can read more in this 2020 paper. https://www.nature.com/articles/s41467-020-15149-4?error=cookies_not_supported&code=5df6700e-6c24-40ba-9192-85e2acb0d513