There are number of articles about mounting SD card read-only, but I only found info, that if needed to write something, first of all you have to remount file system as readwrite. My question is there any way to mount SD card read only AND have file system in RAM, so that everything would look usual (like readwrite), it would be possible to save files without restrictions and after reboot everything would be like before with any changes saved. Like in micro-controllers where ROM does not change. Maybe like in this unclear instructions approach: https://www.monocilindro.com/2017/02/18/how-to-make-raspberry-pi-3-with-raspbian-jessie-read-only/
https://www.monocilindro.com/wp-content/uploads/2017/02/Raspberry_Pi_3_Jessie_read_only.pdf by using smth "boot=overlay" on which I cannot find any info.
Also, I am kind of newbie when it comes to Linux low level background. But I am interested in how Linux manages file systems. For example, if SD card is mounted regularly (read/write), what data of it, of file system is on RAM? Also, when editing files, some sort of buffer is allocated for that, but what if I open a file, which is larger than my total RAM space? And one more thing: like in my first question in this post, if file system will be only read/write on RAM and readonly on SD card, what would be loaded on RAM: all files from entire file system? What if there isn't enough of RAM? Or maybe only used/changed files will be loaded?
Sorry for kind of inappropriate questions, but if you can provide useful links on these topics, I would appreciate them a lot
Thanks in advance