B points out some very interesting questions for you to research. A very thorough list I might add.
There’s a current trend of scaling back within the outside design firms that used to support architects, major museums, and large entities like the National Park Service. About a decade ago, a lot of staff designers were laid off, but then hired back on as contractors if a large project came in. This may have had a lot to do with the major economic downturn then, but it’s now become an ingrained business model. It is very hard to break into that inner circle of designers that have had years and years of networking within the industry.
The way it usually works for us as fabricators, on very large projects there will be an architectural firm at the top. They hire building contractors to build or rehab a building or space. Whether or not they hire the experiential designer depends on who is going into a building. Even if it is a new space, there is already likely a branding system in place and the end client may already have a build-out designer on hire. The exceptions are motif and museum spaces. Depending on the depth of the architectural firm, they may have their own experiential designers on staff or on contract. Or they may hire in one of the studios who then, in turn, staffs the project with their own staff or contractors.
Everything is done by bid. Design firm, building contractor all have to enter into a competitive bid.
On a smaller scale, it may just be an experiential design studio with the chops to do all the detail drawings needed.
Or it may just be a single freelance designer. The thing though, as a freelancer, you are going to need CAD capabilities, whether AutoCAD or VectorWorks (lesser known but more affordable and same file formats.) You are going to have to understand the engineering of sign parts and cabinetry and the materials involved, as well as other materials that can be “repurposed.” You’ll have to be able to project manage a builder who is fabricating your stuff. And you have to understand the limits of the real world. Paper does wonders to hold a design in mid-air. Reality requires visible fasteners. 
It isn’t just fun. There’s a lot to learn.
I highly suggest you check out the SEGD.org site. Lots of material there.
Now for a bit of a reality check.
What is your current employer’s views on moonlighting? How much of what you do on the side could be viewed as competition with them. The minute they perceive you as striking out on your own they may say, “see ya lata.”
I also have a feeling we may be misconstruing your question.
Are you a schooled and experienced designer now? You only say you work in a startup. It’s a very long way up the ladder to getting “logo” work at an architectural level. Sure just about any designer can have a logo made into a sign, but the complexity and quality of the sign will depend on the skill of the designer.
