The best power of social media is that it has enabled users to create different accounts on multiple accounts, gain subscribers on them, and then use those subscribers to cross-promote whatever they like. You can understand it this way — suppose if someone has 20K subscribers on YouTube, 50K followers on Instagram, a solid personal profile and a page with over 30K likes on Facebook, and a decent following on platforms like Pinterest and Snapchat. If that someone is you and you think of promoting your YouTube channel, then you can use all these platforms for cross-platform promotion of your channel.
What you can do is create a high-quality video and post it on YouTube. Now, create an engaging thumbnail and post it on FB, Instagram, Snapchat, etc. to push its reach using your followers on all these platforms. This strategy works so great that you can start a cross-platform promotion and start getting great results within a few hours. All other methods take time to show results, but it gets you instant results. Try it once to grow your YouTube channel.