Welcome to the community @Brrye_Droiu.
Your app can be easily created with Glide. We have similar templates in the template store (free/paid). If you are ready to build just read the documentation, use the gym or a similar template, and then adapt to suit your specifications.
Email with password option is there for you as well as a few other ones.
You can set your app up to assign boxing sessions that customers will be able to book through the application. For example, as long as you know the dates, time and venue, you can get your customers to tap an available session and book in-app.
Train via videos? Provided training videos are available you can upload them to youtube etc and create a link. You can set visibility conditions etc…
Start here: https://docs.glideapps.com/all/