NullKernel

PSA for Linux folders: NVidia driver is currently incompatible with Linux 5.9+ and breaks compute (both OpenCL and CUDA). If, like me, you have the urge to update, Just Say No. https://forums.developer.nvidia.com/t/nvidia-driver-not-yet-supported-for-linux-kernel-5-9/157263

The boot loader (or boot manager) is the specific piece you're talking about - the thing that a PC boots first and will let you select between the different OSs you have installed. GRUB, albeit not exactly a GUI, is the most well-known and well-supported, I believe, and supports both legacy and UEFI setups. Personally, I have GRUB on my laptop and rEFInd - more like a classical GUI - on my desktop (both are Win10, Arch Linux dual boots). Both have kept me very satisfied. In GRUB's case, you have it generate a config during installation - usually it will automatically detect other OSs installed on mounted drives and add them to its config. However, this requires the config to be regenerated when an OS is installed or removed. rEFInd instead loads a driver at boot time and does a quick check of the connected drives for OSs it can boot, removing the need to re-gen the config file. Of course there are many others and all have their pros and cons, so do some reading and pick the one that makes the most sense for you. Ubuntu uses GRUB by default, so you might start with that.