I use a light meter. I own a few Seconic meters. I have one from the
1980's that is still my personal favorite. If I really want to turn heads
on a set I'll pull out my 1975 Luna Pro S - fully analog and works great.
The in camera meter can be fine. It reads reflected light so is less accurate
than an incident meter. The phone apps use the camera phones lens to
read reflected light. Unless you get a diffusion dome. I find them to be
only acceptable; not as accurate as a pro light meter but a fine method
for those on a low budget.
A good light meter is an excellent investment. It will be in your kit for
decades - long after the camera you are using becomes obsolete.
Use it. Over and over and over until you build your skill level.
Get one. My advice is to jump in and spend the money on a good
one because you will keep it in your kit for decades. Lighting and
metering the light to get the exposure you want is a skill. And all
skills are learned by repetition and experimentation. Add a few
small clamp lights and paper lanterns to your cheap halogen lights
and play around with placement.