This thread combines two entirely different concepts:
1. US nationals have a constitutional right to enter the US. The pandemic has never precluded any US national from entry. Similarly, LPR's and certain family members have been exempted from various orders.
2. None of this has anything to do with quarantine and what that involves.
Roughly the same equivalent exists for most countries, especially in Europe, e.g. citizens and those with a residence permit (whatever it may be called) are admissible, but may be subject to quarantine. Again, what constitutes quarantine will differ.
Rather than broad-based statements, the easiest effort is to run the specific parameters which apply to you though TIMATIC. That will display an accurate result for today (which may change tomorrow). If you want certainty, don't plan or book travel until the day you plan to leave.