Can UEFI be accessed by an operating system?

Direct answer is No ,

But you can view UEFI partition via Ubuntu like os via Disks or System Monitor Utilities.

Here are the few screenshots of them on a linux machine.

UEFI partition on SATA drive

UEFI partition on SSD drive

Windows UEFI partition view on ubuntu linux ( Here it is sda1 )

Hope this will help you !

No, you can't directly change UEFI settings when you have booted in Windows 10/8.x/7.

But on a bootable primary partition, you can go to the EFI directory, and see the boot managers like GRUB or Windows boot manager there.

Or there is another option if you don't know how to access UEFI setup on boot time. Open Start menu > Power, Hold Shift then click Restart. You will now restart into WinRE (Windows recovery environment). There you will have an option to change UEFI firmware settings. Click that to access UEFI firmware.

Or if WinRE is not installed or unable to be booted, then insert windows 10 installation media (like a disc CD/DVD or USB drive), click Repair your computer, then choose UEFI firmware settings.

Can UEFI be accessed by modern operatings?

Linux can access UEFI runtime services via CONFIG_EFI in the kernel.

Linux can also easily directly mount the UEFI FAT partition.

Bios interrupts cannot be accessed by modern operating systems.

BIOS interrupts can be accessed just fine by Linux (see e.g. here). For example, for a long time BIOS calls have been used to initialize the X VGA driver (not sure if this is still the case).

However, most of those BIOS calls cannot actually be used, because they require certain data structures and certain system state, which is not present after boot.