Ubuntu is a mix open source source code like the linux kernel and other sources like the one from Canonical
My question is at which point can I see and compile its source code before being stuck on closed source code which will require me to trust blindly an entity
Everything. But that is because it is in your question: "open source code" that means it is "open" and "source".
With that kind of attitude you wont get far. Trust has to start somewhere. How about your change your attitude to: I trust the company of an operating system until proven they can not be trusted?
Accept that Ubuntu itself is open source but that it does have some blobs in the kernel it uses. But that is not just Ubuntu and Debian but also Fedora, Suse, Gentoo, Slack and ANY operating system that wants to be taken serious. If you really want to avoid those there are 9 alternatives that are 100% free. And yes, these are not mainstream for that very reason: basically they are saying you need to abide by what they consider acceptable hardware and will need to purchase acceptable hardware before you can use them.
To avoid installing proprietary software in Ubuntu: this is why we have repositories. The four main repositories are:
Use the 1st 2 and everything on your system is open source, can be viewed and can be altered and compiled. The 3rd one is for device drivers. Most users want their video card and network card to work with all available features so accept the restrictions these come with.
Multiverse is the one to really avoid. That one holds all the proprietary software you can install.