Experienced z/OS developer looking to learn Natural & Adabas — where should I focus first?

Just to add on to Eugene’s comments: for Natural on z/OS, JCL and CICS experience transfer just fine. there are platform differences between Linux and z/OS - character sets, collating sequences, etc that can impact programming. For example, a program might sort on an alphanumeric field, expecting numbers to follow alphas (EBCIDIC order) but on open systems (ASCII or Unicode equivalents) numbers precede alphas.