mp-bgp vrf route leaking

Hello colleagues! I am a bit got lost within mp-bgp and vrfs on fortigates, let me explain in a nutshell.

Let's imagine the schema when we have a "core" multi role switch and several appliances connected as a star topology (or like a leafe spine , but let's skip the redundancy part for now). Links between the core and leafs all L3 and use mp-bgp with VRFs and address family ipv4 vrf plus the rt community. So nothing extra, everything works. It's not the vxlan+evpn example, just a simple campus. Then I would like to connect this core to a Fortigate by the same way for propagating the default gw route and  providing some advanced traffic inspection. The FG can be considered also as a point of route leaking. The confusion is that I couldn't find any examples of this design and how to configure the mp-bgp. It might be the limitation that we can use only an "interface" for each vrf and can use only vrf-lite connection. In some sources I found that it should be organized via vdoms , but why do we need vdoms if we already have vrfs ? It looks like some extra level of abstraction. I've found only one video on YouTube when the guy try to do something similar , but it's not in English and without subs :(

https://youtu.be/vjuXEZ3dbfU?si=Q_PMvfFzRsUwai-_

So can anyone share an example of this config please?  Summary: we need to connect the FG with a L3 switch using mp-bgp and vrfs and provide route leaking. If I am blind and this topic already exists please point out by finger.

thanks!