Best external drive options to install macOS on 2015 Macbook Pro

I have an early 2015 Macbook Pro that has run out of space to be able to update the MacOS on the internal drive. I am currently on macOS Mojave version 10.14.16. As a result I am unable to update software on some applications and I would like to be able to do this. From my very limited understanding I think I can download a newer macOS to an external hard drive and use that as a start up disk enabling me to update my applications? Is anyone able to offer any advice on what kind of external drive I might need that is suitable for this, and where to get it? That is compatible with my Mac, (I have USB and Thunderbolt 1/2 ports) and my current operating system as a lot of them now understandably seem to need an updated one like 10.15. Which is exactly what I need it for in the first place. Any advice would be great, as I feel like I'm looking into the deep unknown :)