Immigration is like darkness. It is the absence of something, not something in and of itself. It is a partial restoration of the state of nature.
The natural state of man is that we move as we please. We stop people from moving as they or we please for various reasons. These stopping points are generally, but not exclusively called borders. Borders are not permanent. They can be moved, erased, reconstituted again. As any observer of Europe will note it can happen quickly and pretty often. Borders do not have to be impermeable. They can have exceptions to the rules. When people are peacefully allowed to cross borders permanently according to the will of the people controlling the border as an exception to the general rule of no crossings, it’s called immigration. Since most borders are controlled by more than one group, the exiting group often calls it a different term than the entry group.
You cannot create a proper system of exceptions to a rule without understanding why the rule exists. But ask people who have strong opinions on immigration why borders exist and more times than not, they haven’t considered the question at all. You could intuit why we have borders by the exceptions but almost nobody does, not even immigration restrictionists. We’re all playing the old game of blind men feeling the elephant and having opinions as to what it is.