First, is there any way to change the default directory of the apache web directory, say, from /var/www/html to /var/www/projects or even simply /var/www?
Sure! Do sudo nano /etc/apache2/sites-available/000-default.conf to edit the default "vhost", and change DocumentRoot from /var/www/html to whatever you want!
Second, what does the 'M' represent in the values of post_max_size and upload_max_size ? Megabyte?
Yes, it represents a value in Megabytes. If you change it to G, it will be Gigabytes, and so on.
Likewise, what if I'll be uploading large (5-12GB) video files to my server? upload_max_size = 12000M?
Yes and no, but see the answer on the next question.
And, if so, I of course must adjust memory_limit so that it is a greater value than post_max_size and upload_max_size, but what does memory_limit actually represent and or refer to?
Yes you would do exactly that. But the memory_limit directive tells PHP how much RAM it can use. You shouldn't exceed the average FREE RAM your server has though. If you have 1GB of RAM installed and tell PHP the memory_limit is 12GB, when PHP tries to allocate more RAM than you have FREE (and not just installed in your system) you'll have problems.
I hope I explained everything in a very clear manner :)
Should you have any other questions, let me know!
i really appreciate that you're taking the time to answer my questions. thanks so much!! i do have two more questions, however.
since i will be uploading large files (5-12GB), but only have 4GB of ram on that old PC, how do i configure this properly? or will this scenario not work at all?
I don't think that it will work. You can upload your files using sFTP, or try to come up with a solution that will segment your files while uploading.
As for Debian, try Ubuntu 16.04, it's an LTS version that will get updates until 2021
hm... i definitely don't want to be segmenting these files. but it's possible to transfer them (large files in whole) to the server with sftp?
i ended up getting debian running by downloading an ISO specific to amd64 and LXDE. thank god.
aww man. i've found legacy drivers but none of them seem to be compatible with deb9.1
*actually, the reason it wasn't working, i believe, was because although i was installing grub on the same hard drive i wasn't installing it as a master boot drive? idk i was scared to click 'yes' to that option because i'm dual booting and i bricked an old laptop a while ago via grub? if i remember correctly, or it had something to do with messing with the partitions and booting stuff, when installing arch