EbonJaeger For the DMS widgets, it's in DMS Settings-->Theme & Colors-->Background Blur enabled, and in DMS Settings-->Theme & Colors-->Widget Styling, I've set Popup Transparency to 50%.
The terminal is Alacritty and for that I needed to create a window-rule in my niri config (as well as set opacity in my alacritty.toml config file):
window-rule {
match app-id="^Alacritty$"
background-effect {
blur true
}
}