If you have a lot of equipment to maintain/want the ability to sort and filter later on, then excel or sheets, as has been mentioned.
I only have 3 vehicles, 2 generators, and a mower that I keep maintenance logs for. So, I just use the iPhone notes app, and have a note dedicated to each piece of equipment.
I enter date of service, mileage/hours, service(s) performed, and anything else I think is necessary to document at that time. I have screenshots of maintenance intervals from my owner’s manual in each note as well.
Keep the services performed in reverse chronological order, so you just open up the app, create a new bullet at the top, and start typing.