use a spray cleaner containing clorox and a tooth brush

I would save the toothbrush for corners. You should be able to find a narrow brush to put on a long pole like this guy is using: grout cleaning tool

Thanks to all of you for the suggestions. Thanks for the idea of the grout cleaning brush and long handle. I would have stupidly gotten down on my knees, mimicking the people in the YouTube videos! Really! 🤪

You can also "paint" the grout. When we sold our last home I was on hands and knees scrubbing 1" x 1" white tiles and grout. Then I went over it with a grout "paint". Looked pretty good after 29 years and 6 people.

I heard today that regrouting is not too expensive. Doing that or painting the grout sounds better than scrubbing.

I could not leave it dirty, that's why I scrubbed it and then used a grout paint pen. Worked great.

I was told that when they regroup, they take the old dirty grout out, vacuum to get all the pieces and dust out, and then put new grout in. I will double Check — I certainly wouldn’t want them to cover the old dirty grout.