Perhaps one through-line is that if IDs are not created by a single source, don't rely on auto incrementing IDs.
The parent/child one is interesting. When I've worked with hierarchical data, I tend to wrap the whole process in a transaction, so it may be many commits (by depth), but one transaction.
Data design outlives programs, and environments by a long time. Circumstances change.
So when I designed the db, there was a single database. But 30 years later we live in a world with smart phones.
Making design decisions because of _current_ circumstances can bite you hard later on.
Reading parent /child. Yes there are ways to mitigate the issue, but it's extra work and code to do so. Ultimately you need the parent id before you can add the children though.