What adapters do I need to connect my old display and usb devices to my 2018 MacBook Pro?
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USB to USB-C:

Amazon.com: Syntech USB C to USB Adapter [2-Pack], Thunderbolt 3 to USB 3.0 Adapter Compatible MacBook Pro 2018/2017, MacBook Air 2018, Pixel 3, Dell XPS, and More Type-C Devices: Computers & Accessories
That allows you to use old USB devices with your USB-C only computer. You plug those on the end of a USB cable then into your computer.

minidisplayport to USB-C:

Amazon.com: USB-C to Mini DisplayPort Adapter, ITD ITANDA 4K Thunderbolt 3 to Mini DP Adapter Cable for Apple New MacBook 2017, ChromeBook Pixel Samsung S8: Electronics

This device allows you to connect your monitor to your new computer (if you’ve not yet gotten a monitor adapter)