You can find a guide on iFixIt, summarized on MacRumors.com: 2018 Mac mini Teardown: User-Upgradeable RAM, But Soldered Down CPU and Storage
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 is (like on the MacBooks) soldered onto the logic board and therefor non-replaceable (you would need to replace the entire logic board).
You’ll need to wait for someone like iFixit to get one and do a teardown to be sure, but since they made a big deal in the presentation of it using SO-DIMMs it is very likely that the memory is user-upgradable. The SSD is probably not officially upgradable, but we’ll have to wait and see if it actually can be removed and replaced or if it’s part of the motherboard.
Edited to add: This review from Tom’s guide confirms that the RAM is upgradable with difficulty (it’s only intended to be upgraded by Apple, not by the user) but the SSD is soldered to the motherboard and can’t be upgraded. https://www.tomsguide.com/us/mac-mini,review-5908.html
Well, this article https://uk.pcmag.com/apple-logic-pro-x-for-mac/118215/hands-on-apple-finally-updates-the-mac-mini says that the SSD is not user replaceable but it looks like the RAM potentially will be.
I just spoke with an Apple employee with the Chat Support and confirm that SSD is soldered and not user-replaceable. so buy the storage you need from the beginning...
however, its an Apple employee, maybe he is trying to sell the priciest version of the product...