I would like to start Muay Thai or Kickboxing but my only option right now is to train by myself. I looked for valuable training material all over the internet and I did find sources to start Boxing, Krav Maga or any other martial art on sites like Udemy, but I could not find any that specifically targets my choice. I know that there are some on YouTube, for example, but the ones I managed to find are mostly not that detailed: since I have never tried any martial art, I would be interested in videos where I can learn the basics of the movements, what can I do wrong or how I should/should not practice etc. Could someone point me in the right direction? Suggesting a quality YouTube channel or anything like that would be much appreciated as well.
There are many wonderful sources on Muay Thai/Kickboxing. Take for example:
You mention they are not that detailed but the issue of learning from this material is not about detail but traing. To practise these art forms requires less detail and a lot more practise in order to develop the foundational cardio, strength, flexibility etc. All great stylists spend countless hours throwing thousands of kicks and punches in order to develop the fundumentals.
With goood fundumentals the more detailed work can be understood which is all down to timing, strategy and the ability to adapt technique. This is impossible to learn solo. It needs a serious commitment to sparring and ring time to bring it to life. There's also a lot of exchanging of ideas with peers and adapting of technique to match the fighters personality (usually best lead by a good coach).
The simple answer to your question is select the popular material to get started but eventually you will need to find a coach/kru or at a minimum a good group you can spar with.