M1 Mac RAM and storage can be upgraded after purchase, but it's not easy - 9to5Mac
OWC 16GB (2x8GB) ram Upgrade PC3-12800 DDR3L 1600MHz sodimm 204 Pin CL11 Memory Upgrade Kit Compatible with Late iMac, Mac Mini, and MacBook Pro Computer at Amazon.com
A-Tech 16GB (2x8GB) RAM for Apple MacBook Pro (Early/Late 2011), iMac (Mid 2010 27 inch 4-Core, Mid 2011 21.5/27 inch), Mac Mini (Mid 2011) | DDR3 1333MHz PC3-10600 204-Pin SODIMM Memory Upgrade Kit at Amazon.com
How to upgrade the RAM on the new 2018 Mac mini | AppleInsider
New MacBook Pro with M1: You can't upgrade the RAM - AfterDawn
MacBook Pro 17" Unibody RAM Replacement - iFixit Repair Guide