Almost everything about all the BIPs is explained on github : https://github.com/bitcoin/bips still looking for a website that explains them differently (simple language, diagrams, history, implementations, etc. which everyone can understand better and appreciate the importance of proposals in Bitcoin.
I am not aware of one comprehensive write-up, but Aaron van Wirdum has a few great write-ups of various proposals in Bitcoin Magazine, the Bitcoin Optech newsletter has covered a number of the more recent proposals in depth, there are a number of talks (and transcripts of those) about BIPs on Youtube from conferences, workshops and Bitcoin meetups. There are a few BIPs that have their own tags here where they are covered more in depth, but again, I don't know about a single resource that is just focused on explaining all the BIPs. Maybe someone should start one? :D
Until then, our site here is a great place to ask all your questions about any BIPs.