The Ubuntu release upgrader tool will upgrade you to the next release (ie. 16.04 -> 16.10), or to the next LTS (18.04). More recently it's also supported to the next non-LTS in the next-LTS development cycle (ie. 20.04 would allow upgrade to 21.04 after 20.10 had reached EOL, 21.10 after 21.04 reached EOL) but that wasn't yet available at 16.04 (not that it matters as 16.10, 17.04 & 17.10 are all EOL!). Other upgrade paths are unsupported and not QA tested against so all checks are those you perform yourself.
You didn't specify if you're talking about Desktop or Server (it very much matters), but you can upgrade via re-install using the repair installation options of Ubuntu installers to allow a quick/easy re-install of a different release (it was intended for the same release; but no version checks are performed) which I do somewhat regularly... but large jumps still require homework to ensure it'll work perfectly (ie. package review as you may have missed steps done in the interim for apps you rely on and this may cause issues; this is package specific and not release specific) The re-install process I'm talking about will also re-install your manually installed packages (if internet is available for download during install & the packages are available for your new release), but as system directories are wiped - server configs will be lost thus not useful for server re-installs; being intended for desktop re-installs.
The upgrade path you suggested (18.04 to 20.04) is a supported path as that is from one LTS release to the next LTS release... What you're asking for is very different.