Fonts are configured in a terminal emulator. I don't remember seeing a way to specify a font from an application and if there is one, it's unlikely to be possible to use more than one font at a time (their metrics would have to match or be adjusted on the fly in order for this to have a chance to work).
If JetBrains Mono has some icons, JetBrainsMono Nerd Font may use different (potentially bigger) glyphs for them. Otherwise you would probably have to combine fonts or edit one with FontForge or a similar application to scale/replace glyphs.