In Ventura, open System Settings and navigate to the Keyboard section. Click “Keyboard Shortcuts…”. Click “App Shortcuts” in the sidebar. Makes sure there is an “All Applications” section, and add one if not. Under “All Applications” add a shortcut for “Semibold” (be careful to spell this exactly), and the keystroke you want. You might want to also add a shortcut for “Bold” and change it to something other than ⌘B.
The shortcut you add for Semibold will work in any app that uses the macOS framework for assigning fonts and styles to text. It will not work in apps such as Obsidian that do not use that framework. The shortcut will also only be effective with a font that has a Semibold style.