Materials Saw It On YouTube

All right, this will be a bit long...

When you do your research and planning, make sure to always find several sources. Youtube can be great, but never stick to just one tutourial or one channel. Use it as a starting point or inspiration only.
Once you have an idea of what you want to do, research the different elements separately and make sure you understand the materials and process.

You honestly need nothing fancy to make a full scale blueprint. Just a piece of paper large enough and a pencil will do just fine. If you need the doll to have some specific measurements (to fit in with an existing doll, for example, or to match a specific scale) then a ruler or measureing tape is also good to have. But you need nothing more than that.

There are many different sculpting materials and they need different core materials. If you use polymer clay you need a core that can be baked with it, such as tin foil. If you use air dry clay floral foam or styrofoam will work and is easier to shape, but things like crumpled paper covererd in masking tape will work as well. For epoxy clay you need someting that won't stick permanently. You may need to make a few tests.
And that bring us too: Test your materials out before comitting to one. Some of it is a matter of personal preference, but other factors are in play as well. You mention that you live in an apartment, do you have a proper work space? If not, then perhaps looking fo the least messy and least toxic materials is a good idea.
Du you have a plan for how to manage dust? What about disposing of waste?

Spray primers are used mainly to get an even coverage and because it's faster than painting a primer on. Depending on what you need the primer for you can probably get away with a brush on primer, if you need one at all.
If you prime for casting, then a spray primer is a good idea since it will cause less work to sand it smooth. There are also spay on fillers/sanding base primers, that can be used before the ordinany primer to even out the surface if needed.
If you prime an art doll for painting, then you may be able to get away with a suitable base coat to help the paint adhere to your sculpting material. A brush on one is probably good enough then, since the brush marks of the final paint probably will obscure the bruch marks of the primer. A doll in porous material such as paper clay or papier maché can be painted with acylics straight on the doll, but a primer can help make it a bit more durable.

How far do you intend to take the project? If you intend to take it all the way to casting then spray primers will be the least of your worries. Polyurethane fumes are toxic too and so are some types of silicone. I did start out in an apartment and while I did manage I can't recommend it. This is workshop stuff, not cafts to do at the kitchen table.

If you are itching to get started and don't want to wait until your situation is more suitable for doll making, then perhaps start with an one off art doll in paper clay or some other fairly safe material? That way you can get an idea for if this is something you want to do and if so, then you know it's worth hunting for a place to do it.

Read through some of the project journals here. Look for two things.
1. For any project that has been left unfinished for a long time, at what stage did the project stall? Consider why and if that is a potential problem spot for you as well.
2. For finished projects, look for what materials has been used and if there are any worktable pictures, what kind of conditions are people working under and what equipment do they use. Is that something you are able to do and comfortable with?

And finally: Test everything!
Make material tests to see how they behave. Test materials in combination with each other (and with your tools and protective gear) to make sure there are no bad reactions. Test materials to find out about curing times so that you know how long you have to preform a certain task. Read up on your materials to know what to expect and then test it out to see if reality agrees. Sometimes variables such as temperature, humidity, drafts and contact with other materials can cause things to behave in unpredicted ways.
The last thing you want is to realize that you've covered your doll in a primer that will cause your silicone to not cure or your paint to peal. Or that your new type of gloves causes issues with the resin curing and just ruined mold. Or that your sculpning material is reacting with your core material and is melting through it.

 

Hi! So @Lillith is way more suited to answer and honestly loved reading the answer and felt like I learned so much!
just wanted to add on— while I do have MSC and am familiar with its feel, I have a bird… so spraying at home is a no for me. I tried this matte varnish and was surprisingly pleased! I’m not a great painter, but every time I use it, I can’t find any unevenness & I love the way it feels!! Also feels great to paint on in my opinion. I’m not sure if it is a great option or would work for everyone in every situation, but I’m happy with it.

But as usual— definitely try it inside the headcap or somewhere discrete to make sure you like the way it works & no damage or anything occurs.

 

If you have a cat, then any kind of sanding is also going to be pretty much out of the question at home. Pets and crafts don't mix. Some dust will get away, even if you are careful and do your best to limit it. It will land on the floor, the cat will walk, sit or lie down on the floor and then wash herself and lick her fur, ingesting the dust.
Even the "non toxic" scultping materials such as paper clay are only safe when used as intended, they are typically not safe to be eaten. Also keep in mind that the producs MSDS (material safety data sheet, sometimes shortened to SDS) is written with humans in mind and sometimes animals react differently to chemicals.

I don't mean to discourage you, you may be able to find some sort of a workaround that will work in your situation.

The varnish and the MSC mentioned by @strawberry_sea are great for face ups on finished dolls, but may not be what you need for finishing an original sculpture for painting or casting. Make sure to research your options depending on what your primer needs to do for you.