Debian installer puts GRUB on main disk without asking

Hi all,

I wanted to install a Debian Linux system (with LUKS) on the remaining space left by a Windows 10 install (with BitLocker and Secure Boot), without messing with the Windows boot or the main disk (nvme0n1 below) boot sector . As I needed an unencrypted filesystem to put the Linux kernel+initrd and also the boot loader, I decided to use a 1 GB pendrive (sda below) for both things

This is how the Debian partition manager looked like (I guess that 'B' means boot, but I don't know if that implies that GRUB is going to be installed to the main disk...):

Encrypted volume (nvm0n1p4_crypt) - 104.8 GB Linux device-mapper (crypt)
    #1          104.8 GB        f   ext4        /
/dev/nvme0n1 - 512.1 GB
                1.0 MB              FREE SPACE
    #1          536.9 MB   B    K   ESP EFI system partition
    #2          134.2 MB            Microsoft reserved partition
    #3          406.6 GB            Basic data partition
    #4          104.9 GB        K   crypto  d10root     (nvme0n1p4_crypt)
                335.4 KB            FREE SPACE
SCSI1 (0,0,0) (sda) - 1.0 GB Mass Storage Device
    #1 primary  1.0 GB     B    F   ext4        /boot

Later at the boot loader part, it didn't ask if/where to install GRUB. There was some message about probing or detecting partitions and at Alt+F4 screen I saw something like "grub dummy" but I can't remember exactly what the command was. Then it started installing packages

After completing the installation and rebooting, if the pendrive is not inserted I see a GRUB screen, which is what I was trying to avoid, and also it can't boot Windows or Linux ("minimal BASH like line editing is supported" message). If the pendrive is inserted GRUB works fine.

Linux boots fine but Windows' BitLocker didn't like it and wouldn't boot, it asked for the BitLocker recovery key, then it "fixed" the boot. After that fix, and rebooting, both Linux and Windows boot fine.

For the next time, what would be the correct settings to install both GRUB+boot files on the pendrive and not touching the main disk boot at all, or as an alternative, install both GRUB+boot files on the main disk and not need the pendrive to boot? Or even not using GRUB and just using EFI boot manager?

Thanks