The Commodore PC-1 is the most compact of the PC compatible computers from Commodore. It doesn't have any XT/ISA slots for expansion cards. However, it does have a proprietary, but nearly similar expansion port on the back on the computer.
Youtuber MIGs Yesterchips (who, by the way, also runs a retro computer museum in Germany) came up with an idea and then built, on his channel, a prototype of an adapter that would allow you to connect an XT expansion card to your PC-1's expansion port.
Heavily inspired by this prototype -- and of course with the permission of MIGs Yesterchips -- I have built an open-source, finished PCB that you can either buy as a kit or ready-to-use, or you can get the source files and have your own PCB made at your favourite PCB manufacturing company.
This adapter, the PC1-XT, allows you to use expansion cards such as the XT-IDE expansion card from The Glitch Works, which, in turn, lets you connect a harddrive, DOM or a memory card to act as a harddrive. Something that the PC-1 also did not come with.