The Source0 field provides URL to the upstream software source code. It should link directly to the version of software that is being packaged. In this example, we can use https://example.com/bello/releases/bello-0.1.tar.gz.
The code seems good. Is the rpmbuild automatically downloading sources from a remote server?
It is a long time I'm not building RPM (cloud came in the meanwhile :))
A .spec file can specify it's own Sources on a remote server. As such, the $SOURCES is optional with a warning.
https://rpm-packaging-guide.github.io/