Hmm, what do you mean by session and channel retention? Basically just how long they watch your video for, and if they clicked it in the first place?
There's 3 types of retention the algorithm calculates:
1. the retention % of the video = how long the viewers watch that particular video. This ranks the videos on your channel.
2. the contribution to the overall viewing session your video makes, does it encourage the viewer to stay on Yt and watch more, or does it make them close their browser and kill their Yt session, and does your video start their session. The worse thing that can happen is your video kills their session.
3. your overall channel retention, which consists of two main scores, which are the average and ranked retention for your channel. Say your channel average retention is 50%, that may be great compared to kids channels that have an average of 35%, but it may be terrible compared to other channels in your vertical which may have an average of 80%. If you want to get in 'suggested' and 'browse features' as traffic sources (but not search, that's separate) your overall channel retention is ranked to all other channels in your vertical and you are allocated a ranked position in your vertical. This is the primary decision factor the algorithm uses to determine which channel to place in available spots.
So how the algorithms works is beautiful and simple (retention wise):
1. each video on your channel is ranked according to retention
2. your channel is ranked in your vertical according to retention
3. when suggested spots come up, the algorithm chooses higher ranked channels to promote on higher subbed and higher ranked channels (and conversely lower ranked channels to promote on lower ranked and lower subbed channels)
4. when browse spots come up, it chooses higher ranked channels to promote on prime real estate
5. you get allocated traffic based on your channel rank, this ranking continues until you level up or down
This is the reason why is you go to a top kids channel, like RyanToysReview, you will never see in the suggested spot a video from a channel named "KidsToyReviewsMeToo" which has 23 subs and 14 views on it. Since it's a new channel its video retention will likely be 15-25%, channel retention probably 20% and the channel has rank 1,534,667 / 3,982,999. But you might see suggested videos from HaileysMagicalPlayhouse or DCTC or ToysAndMe, which would have ranks in the top 10 of kids channels.