How can I grow spinach & lettuce without it getting all dirty?

try a hot house version to grow them in so its protected from rain.

Use some landscape fabric or plastic sheeting over the soil.

Hello Would you consider using hydroponics?

https://homeguides.sfgate.com/grow-lettuce-using-hydroponics-69112.html

Dirt is part of the process. Now you know what farmers go through. Mulching the plants will cut down on the dirt that splashes up on the leaves. All you need to do is know how to clean leafy greens. These will help.

https://livesimply.me/2015/07/17/prep-day-how-to-wash-and-store-lettuce-kale-spinach-and-other-leafy-greens-in-advance/

https://www.finecooking.com/article/how-to-stem-and-clean-fresh-spinach

https://www.thekitchn.com/stop-washing-your-lettuce-kale-and-spinach-the-wrong-way-248347

Another option is to grow hydroponically.

I have never tried spinach, but I grow all kinds of lettuce in my Aerogardens (hydroponics). Quick, long growing season, clean to beat all bands. Never even rinse the leaves off so dressing stays on them much better. If you've never tried aa Aerogarden, go for it - I have four!

Taller raised beds can be helpful. I have less issues with my plants getting dirty from rainfall when the beds are 1 or 2 feet off ground.

You can see an example of these raised beds here

https://www.gardenupgreen.com/2018/10/creating-a-new-garden-space.html

Dirt is best. Just plan to wash thoroughly before eating it.