As quite correctly stated above, partitioning can and does work.
So too does configuring macOS as a guest in a hypervisor.
And so too does configuring and using and potentially booting from external storage; of using one or more external HDDs or SSDs.
One huge caveat with storage partitioning: corruptions can corrupt some or all of the partitions, and recovery can require reinitializing the whole HDD or SSD and then re-partitioning and restoring per-partition backups. More than a few folks have encountered partitioning bugs and partitioning corruptions. For those with no backups or no current backups of one or more partitions, that does not end well. There are lots of sad stories with partitioning.
TL;DR: When making any changes to partitioning, have backups of all partitions; of any with data that you care about.