How to track memory usuage?

I’ve got 1 VPS with not too many WordPress sites that I’ve built and manage. Specs are 6 x 2.80GHz CPU / 12 GB RAM / 192 GB SSD Space / 8 TB Bandwidth. Every time I check in WHM, it’s saying memory levels are around 28%.

Within each WordPress website, is there a way to see which of my website executions specifically generate the allocation on my server environment, as well as which processes for each WordPress site so to nullify said execution? My thought is that it comes down to which plugins are being used, but I have no idea on how to investigate it. Sometimes I’ve been seeing this: https://gyazo.com/5c4d7282d9b67db54bba356e54fa6987