I have worked with agile for the last ~15 years or so, in small and big companies, starting from scratch or in places where agile already was implemented.
First thing first, Agile is not a silver bullet. Its not going to work for every company, because every company has a different culture/mindset about it should work the best. Unless you manage to revolutionize the culture, you are not going to be able to apply agile in it if there are people against it.
Second, the definition of 'agile' will differ from company to company. For one company agile is having sprint plannings, daily standups, refinement meetings with planning poker, retrosepctives, the whole shebang. For others, 'agile' is having a kanban board with some stickies and doing a daily meeting on front of it to see what progress has been made. Each company can, and SHOULD, implement 'agile' in a way that suits their culture, their pace of development.