Difference between PPP, QMI and AT commands

During the past 6 days, I had the chance to interact with an LTE modem (https://sixfab.com/product/quectel-ec25-mini-pcle-4glte-module/) , I managed to set up 4G Internet access on a Raspberry Pi using "QMI". But I still have A LOT of questions, I read many tutorials, articles and even articles from books but I'm still confused, I hope you have time to clarify things for me and for future readers: - How do PPP, AT commands and QMI interact with each other and with LTE? - What is the role of each of them? Do we need them all? - Can you explain what happens (in details) when establishing a connection or rec/send data from the internet using these protocols? schemas will be great. - I understand that PPP is used to connect the modem to the ISP, why do people say that it should be replaced by QMI? - In this response https://unix.stackexchange.com/a/168690 we recommend using the wwan0 interface over tty for more speed, but all data shares the same only USB connection, right? - if the max serial speed is 115200 bps, how do modems reach 150Mbps ? do they have buffers? - According to https://www.quectel.com/UploadImage/Downlad/Quectel_WCDMA<E_Linux_USB_Driver_User_Guide_V1.8.pdf (page 8/47) the model provides many interfaces and they say that wwan* interface can be used as USB network adapter, what is the difference between wwan* and /dev/ttyUSB* ? - Can you suggest some articles or books about that?