Today, developers are using languages as tools to solve problems, and their main languages are sometimes not effective for particular problems. Therefore, they are starting to learn new programming languages. There are some methods you can use to learn new languages.
There are programming courses which are opened by universities or organizations. These type of courses mostly offer you a certificate that you can get. You should actually go for some place like classrooms and attend this lectures to get these certificates. These courses can be free, but some courses needs to be paid. Also, there will be progression in time for lecture contents.
These type of courses offers video lectures that content is already completed. You can find free and paid online courses like in Udemy, Coursera and Youtube.
You can find documentations for your new programming language online. Also, they mostly provide some basic tutorial to use the new language to get you started.
You can learn new languages by using books. Books are great if you don't like to use screens for this job. Content of the books are more detailed, so you can learn some interesting details that you may not learn using online videos.
To sum up, you can learn new languages from various sources, but you should keep practice in the new language to memorize and master your skills.