Standing up to read in the synagogue like #yeshua did in Luke 4:16-17 wasn't something anybody could do whenever they felt like it. There is generally a strict order to these things. That the synagogue rulers cooperated with him, shows that he had some recognition as a teacher or dignitary of some kind.

In v18, he quoted from Isaiah 61:7, 42:7, and 49:8. He probably read more than what is reported, but these were the relevant excerpts. His commentary is really what blew the listeners away.

I suspect he said more than is recorded in v21. As with the preceding reading from Isaiah, his commentary is likely also summarized by Luke.