Nowadays you draw it in your phone/computer. In the old days you looked up by "radical" and number of strokes in large tables.

When you finish middle school, you're supposed to learn most of kanjis used in real life.[0] At that point it's pretty rare to encounter kanjis that are completely foreign to you.

But there's electronic dictionary on which you can draw to search for kanji.[1] I used one of these in high school.

[0] https://en.wikipedia.org/wiki/J%C5%8Dy%C5%8D_kanji [1] https://www.casio.com/content/dam/casio/product-info/locales...

The easiest way is just using OCR which for example the Google Translate app lets you do. After that you can search by drawing it or search by the radicals that you recognize in it.