Masking

library(terra)

fire <- rast("data/AnalysisReady/burn_severity_ba4.tif")
dem <- rast("data/AnalysisReady/elevation.tif")

# Set fire values outside perimeter to NA
fire[fire == 0] <- NA
plot(fire)
# Mask the dem to only include values within the fire perimeter
dem_m <- mask(dem, fire)
plot(dem_m)