Probably start with wlroots if you want to target multiple Wayland desktops. There are multiple compositor implementations now, so grabbing a framebuffer isn't

Probably start with wlroots if you want to target multiple Wayland desktops. There are multiple compositor implementations now, so grabbing a framebuffer isn't quite as easy as it was in the 90s. You should check out Flameshot and Spectacle if you want feature-complete screenshot apps on Wayland. If those don't suit your needs, then you should consider a custom solution.