How to properly install and configure grub?

Question

GRUB

Greetings.
I have the following partitions on hard:
/dev/sda1 - the partition "reserved by the system", which was created by Windows. It is labeled "boot".
...
/dev/sda3 is the partition on which windows 10 is installed.
...
/dev/sda6 is the partition on which ubuntu is installed.
The task is to be able to select the OS when turned on.
After installing Ubuntu, of course, Windows just continued to boot for me. I installed from liveUSB to /dev/sda1 grub, now I get GNU-Grub terminal on startup. How can I do now so that I can normally choose which system to boot? I suspect it's grub-update or grub.cfg...
And yes, by manipulating the grub command line, Ubuntu can be started, but Windows cannot, grub does not see it. Maybe due to the fact that the Windows bootloader was on sda1, I replaced it with grub :/
I can rub the whole hard, it's not a pity, just explain how to install it correctly in the first place. I googled, and a lot, but pretty general information is given everywhere and everything goes very easily and simply, not like in reality.

Answer 1

In what mode did you install Ubuntu?
I suspect that the old-fashioned BIOS / SCM. Windows is in UEFI mode.
So UBUNTU had to be installed by booting from the installation media in UEFI mode