Is memory or storage in the 2018 Mac mini replaceable by the user after purchase?
You can find a guide on iFixIt, summarized on MacRumors.com: 2018 Mac mini Teardown: User-Upgradeable RAM, But Soldered Down CPU and Storage
Memory: Yes*
Apple specifically mentions on the Apple Website that the Memory are SO-DIMM sockets (2666MHz DDR4 SO-DIMM).

Mac mini features high-performance 2666MHz DDR4 SO-DIMM memory

*However, the warranty is voided if upgraded by the user as the modules are no longer directly accessible from the bottom but instead involve removing the antenna plate, fan and mainboard.
Storage: No
Storage is (like on the MacBooks) soldered onto the logic board and therefor non-replaceable (you would need to replace the entire logic board).