The weird installer: Internet connection, UEFI/BIOS message and disk mistake.

Hello,

I tried to install Debian 13 (UEFI) on my computer. Everything was great,
before:

1. I have a USB Wi-Fi adapter. I turned it off bevause it is hard to
install Wi-Fi drivers. The computer was completely had no Internet.
While the installer checking DHCP, it said that connecting was failed (I
planned to connect the PC to the Internet later).
If it failed, is it possible to connect to the Web with Ethernet after that?

2. My computer has UEFI instead of BIOS.
I turned off fast boot. When the installer said: "If you have different
OSes on your PC, why don't you use BIOS mode?". It confused me, because I
wanted to delete Windows and leave Debian on it.

3. I have two hard disks (maybe SSD). In the box which was asking me which
disk I want to use. There was SanDisk Cruzer Blade, it was my USB drive
which was used to install Debian. There was also only one (I had two of
them!) hard disk that had 500 GB. I never had that weird disk.
One was  ~100 GB drive which has Windows 7 installed, another drive was D:,
it was empty but had ~300 GB.
Maybe it means to use the hard disk that I never had?!

Thank you.