Versions are specified within the requirements.txt, if that's what you mean. Upgrading can be done by installing a later version of the package and rewriting the requirements file.
I suspect my use cases haven't been as complicated as some of the others listed in this thread, which may be why I've never felt the need to look into poetry and others.
I know what you mean, but as soon as you have transitive dependencies it doesn't work at all. At that point you can't reproduce the same state of your venv at a later date because some minor version of a dependent package could change and maybe break your build.
I suspect my use cases haven't been as complicated as some of the others listed in this thread, which may be why I've never felt the need to look into poetry and others.