Some advantages of YouTube videos:
1) Save money – they’re free
2) Learn at your own rate – freedom of scheduling
3) Get multiple perspectives by watching multiple teachers
4) Get a sample of someone’s teaching style/personality/singing before paying for their online course and/or lessons
Some advantages of online courses:
1) Save money over taking multiple lessons
2) Save time spent searching for content on YouTube
3) More comprehensive, organized content than on YouTube
4) Learn at your own rate – freedom of scheduling
Some advantages of apps:
1) Save money – some apps or parts of apps are free
2) Get real-time feedback on pitch
3) Good for ear training
4) Learn at your own rate – freedom of scheduling
Some advantages of voice lessons (with a good voice coach):
1) Maximum improvement rate:
a. training tailored to you
b. a good coach can quickly hear what’s “off” in your technique and suggest ways to fix it with real-time feedback – including issues you might not have otherwise known you had if you aren’t recording yourself/don’t have a great ear
2) Added motivation to improve:
a. weekly/every other week lessons give you a deadline to work towards for your practice
b. being held accountable – having to admit to someone if you haven’t practiced
3) The joy of working with someone who’s rooting for you and gets joy in seeing you improve
4) Can help with subtle refinements of your singing technique to a professional level
Other options:
1) Video feedback services such as from Robert Lunte (https://www.wisio.com/robertlunte) or Kevin Richards (https://www.wisio.com/KevinRichards#) where you send a video of you singing and they send you feedback on what to work on. This is less expensive than a full lesson, and can be a great way to get personalized feedback and suggestions for exercises for your particular vocal issues. You won’t get real-time feedback, though. This may be a good option if you’re motivated, but have limited funds.
2) Online forums to ask questions and ask for feedback such as https://www.themodernvocalistworld.com/…/14-review-my-sing…/ and https://www.facebook.com/groups/TheVocalistStudio/ – these are free, and may help you when you’re stuck. The quality of feedback you get may not be the same as when paying an expert to assess your singing.
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Some of the factors in which of the above resources you may want to use to improve your singing are: 1) your self-motivation level, 2) how much time you have, 3) your funds, 4) the quality of your ear.
I’d personally advocate for a combination of the above options. Have an online course as a go-to resource, and to get you generally started in the right direction, use singing apps to help with pitch and ear training, do YouTube searches to learn different exercises and takes on things that may help you improve faster if you’re consistently not getting something, be part of an online forum to ask questions when you’re looking for a quick answer, and work with an experienced voice instructor to help you design training routines, give you real-time feedback, and keep you on the right track. If you don’t feel you can afford voice lessons, periodically use a video feedback service, such as whenever you feel like you’ve reached a plateau.
Thanks for reading, and I hope this helps you find your own optimal path to improved singing!