A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
If you don't have access to a migration tool like Sharegate, because the underlying functionality of SP hasn't changed, the easiest option is to save lists as templates and then upload the template to the target site's gallery. Not useful if you have large lists above 50mB due to Msft's rather pathetic limitations. In that scenario, I'd save the list as a template without content, recreate it in the target site, connect the source list to an Access database, and then sync the new list with that database. I'm not sure if the links to attached documents will get updated though.
It's so typical of Msft that they dumped a new version of Sharepoint on us but offered absolutely no useful migration options. Other companies like Sharegate were able to come up with a really good option, so it's pretty telling that Msft didn't bother.