The zlib1g name has something to do with the libc5 to libc6 transition that affected all of Debian back circa the year 2000 I think. The libssl-dev thing is be

The zlib1g name has something to do with the libc5 to libc6 transition that affected all of Debian back circa the year 2000 I think. The libssl-dev thing is because (usually) you can only have the headers installed for a single version of a library. So you get libssl-dev. But you can have multiple binary versions installed, to support other packages that may have been compiled with older versions of the library with incompatible ABIs & that haven’t been recompiled or updated to the latest version yet. So you have a -dev package that depends on the latest binary package. Sometimes, you have headers that live in their own versioned directories, so you can compile against older library releases (maybe there’s an API change & not all your packages have been updated to the new API yet). Hence the libssl1.0-dev, which put headers for the old libssl version in a subdirectory in Debian stretch (I think?) so they didn’t interfere with the ones for the new version.