I am using Windows 10 and I'd like to install Ubuntu as a secondary OS (I want the first option in the "boot-up" screen to be Windows). I am a noob so please bare with me. A couple of years ago I somehow managed to do this, but I was only using one HDD at that time and didn't run into these issues.

Windows 10 is installed on an SSD on my laptop. I also have a HDD inside and I shrunk this partition and freed up 50 GB to create a partition specially for Ubuntu on my HDD.

I flashed a USB stick with Ubuntu on it and I am unsure how to proceed.

My problem is that inside the Ubuntu installer I don't know whether I should choose Primary or Logical and which Mount point is correct. I tried Logical (which is the default option) and "/" as a mount point and the partition got created as 'Ext4', however, I don't know what option I should choose for "Device for boot loader installation". I tried /dev/sda as suggested in another thread here but the installer said "No EFI system partition was found" and it warned me that the installation might fail.

In msinfo32 it says the BIOS mode is Legacy, if that is of any help. Also, here is a picture showing all the partitions available (the last one is the free space that I left after shrinking the HDD which isn't visible in this picture, but it's the 50 GB free space):

Image of available partitions as shown in the Ubuntu installer

Other steps I've done after reading How to Install Ubuntu on separate hard drive in a dual boot With Windows 7?

I shrunk my HDD using Disk Management inside Windows. Also, Ubuntu Live USB gives me two options only: "Erase disk and install Ubuntu" or "Something else" (I chose "Something else"). After I created a partition from the 50 GB free space and chose /dev/stb as "Device for boot loader installation", it still tells me: "No EFI System Partition was found. This system will likely not be able to boot successfully, and the installation process may fail. Please go back and add a EFI System Partition or continue at your own risk." So I created a 256 MB EFI partition from the 50 GB free space as suggested in one of the related threads and the installer accepted that as a device for boot loader installation and Ubuntu was installing, but at the end it said: "Executing 'grub-install/dev/sdb5' failed. This is a fatal error" (sdb5 is the EFI partition I made). It prompted me to report the error to the developers but I didn't do that. I restarted my PC and it straight-up booted into Windows. No option to switch to Ubuntu. This is one of the most frustrating things I've ever done and I don't understand what I'm doing wrong.

Thank you in advance!