For the restore operation, according to the documentation, you can go as far back as a 2005 backup and restore it to 2019. As to the in-place server upgrades, you've already got the documentation. After the restore, your database will be in an older compatibility mode. This will keep some operations working as they used to while you test the upgrade. After the tests are successful, you can change the compatibility mode and get all the benefits of the upgrade (and possibly hit some problems, especially with cardinality estimation). I strongly recommend you take advantage of Query Store as a tool during your upgrade processes. I have a blog post about it here.
That's about it. Strong recommendation, plan for an extended testing period if you're upgrading from 2012 or less to greater than 2014. When 2014 introduced the new cardinality estimation engine, some queries on some systems suffered quite a lot. That's mostly likely to be the source of any serious problems. Although, you may hit issues anywhere along the way. It's hard to know for certain with tons more information about your systems.