Not how it works. Each kill gives you points towards each engram, yellows give more points, bosses give even more points, hit a point target & an engram drops. Each pool has different points, & targets, each activity likely has different totals & each pool has different loot thst can drop from engrams. Exotic engrams are a master account drop if you like, where everything gives points that you play, probably more points given the harder the content you're doing. It's far more nuanced than that & it's confused by old content using old numbers, plus spaghetti code causing issues. Best to forget about it & play the game 👍

It's a weighted tally and because more difficult enemies are weighted higher that's why they'll drop them more often breaking those thresholds more frequently. The metric for exotics was once known, but I don't think anyone has remapped it in a long time. There are RnG procs as well, but you can absolutely farm kills to guarantee exotic engram drops via attrition. It was 2k something or 4k something, don't trust that, I can't remember. Every enemy is weighted so like an easy delta redbar would be worth 1. Once you hit the threshold an exotic engram drops. Edit: Ok, I did some research, I couldn't find the exotic threshold data I remember but I think I know it now. It was approximately 2.5x the prime engram drop rate which was 1800 weighted by enemy type. So, 4500 for an exotic guaranteed via attrition.