Problems with FileZilla, CPanel and themes installation

Hi! I have some questions regarding FTP clients, CPanel, themes and plugins installation. I’m just posting all of them here for convenience reasons and also because they are, in some way, related:

1)While trying to install FrameMarket I faced lots of transference problems following the installation instructions provided by WPMU DEV here https://premium.wpmudev.org/project/frame-market-theme/installation/

I created a new topic in the forums, reporting my issue (link to the topic: https://premium.wpmudev.org/forums/topic/cant-install-framemarket#post-187706) and a guy told me “Please do NOT install using the instructions. Ignore them. Forget FileZilla.”. Now, my obvious question is: If the installation instructions provided by WPMU DEV are wrong, why these instructions are there?

2)I’ve already installed WPMU DEV plugins following the instructions provided by the site itself and didn’t have any problem. About FrameMarket, that was the first time I installed a WPMU DEV theme and I imagine that the installation instructions are basically the same to all themes here. Does the fact that the FrameMarket instructions are wrong mean that the installation instructions to all other themes from WPMU DEV are also wrong?

3)Why am I seeing the BuddyPress Default theme as one of the themes options of my site if I didn’t activate this theme to my network?

4)What’s the difference between using CPanel to manage my files and use a FTP client (I use FileZilla) to do the same?

Thanks!