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There are 59 National Parks in the US, including Joshua Tree National Park.
There are also numerous "National Monuments", including Cabrillo National Monument.
National Parks, with one or two exceptions, have one thing in common - they are big! Ranging from very big, to very, very (very!) big! In some cases transport is available within the park - at least for part of the year. For example, Zion National Park in Utah is closed to (most) cars for part of the year, and buses are used to shuttle visitors from one end of the park to the other.
However in general, there is no transport available within the parks other than any that you bring with you - normally in the form of a car/motorbike, although for some parks a pushbike would be suitable.
Specifically for Joshua Tree National Park, not only is it very large, but it's a long distance from any reasonable size population center, and from the nearest airport (Palm Springs). Even just driving from one side of the park to the other is over 60 miles and will take well over 1 hour each way! Once in the park there are various hikes available, ranging from a few hundred yards (such as through the "cactus garden") up to over 10 miles. I've hiked several of these trails, such as Lost Palms Oasis, Lost Horse Loop and Baker Dam (click on the links to see photos/maps of the trails).
National Monuments on the other hand are generally much smaller, such as a single building, statue, or a small area of land. For example, the Status of Liberty is a National Monument. Generally these are small enough to walk around, although in some cases they may require transport to reach them.
Cabrillo National Monument certainly is small enough to walk around, and being very close to San Diego there is public transport available to the park (details on the NPS websites).
Many National Parks have entries fees, as do some National Monuments. You can find the costs for each park on the NPS Website. If you are going to be visiting a number of parks I strongly recommend the America the Beautiful annual pass - It costs $80, but allows free access to all National Park and Monuments for an entire year. Otherwise you can just pay for each part individually - just keep your receipt as you can normally use it to enter on multiple days.
The National Parks website is excellent, and contains details for all National Parks and National Monuments.
Upvote:7
It depends on the park. Some of them have accommodation within, others just outside or in nearby towns.
Let's take Joshua Tree National Park as an example, since you're looking at going there.
You can fly in or take the train into Palm Springs, nearby, or drive. There are specific entrances to the park, where you usually pay a fee for conservation and the like. The north entrances to the park are located at Joshua Tree Village and the city of Twentynine Palms. The south entrance is at Cottonwood Spring.
Then it will, as I mentioned, cost you to get in. This is done in a variety of ways - The Joshua Tree National Park Annual Pass, $30 for 12 months; vehicle entry, $15.00 for 7 days; walk-in entry, $5.00 for 7 days.
Now, getting around the park. As you mention, some are big. However, many people will park a car near the gate and walk in, as it is cheaper and perhaps the part they want to see is close to the gate. For example, Yosemite - the first night I was there, we wanted to see the giant trees just inside the entrance, and it was walking distance from the west gate we entered at.
You can drive around Joshua Tree - as indicated by the vehicle entry charge. There are main roads, as well as several dirt roads. There are also many hiking trails - you'll want to make sure to take lots of supplies. Finally, you can mountain bike around the park as well - check at the entrance for maps and details.
Usually most parks will have highlights that you want to see (eg the Half Dome at Yosemite, or Yosemite Valley). They'll be on maps, and you might drive to a point, and then walk a trail for a while, say, to get to a particular viewpoint or waterfall or similar.
Incidentally, at Joshua Tree, there are campsites within the park, if you wished to do so. If you want to stay in a hotel, you'll have to stay outside the park.
Much of this information I gleaned from the Wikivoyage page on the park, which I recommend checking out if you're planning on going some time soon.
Another fine source of information is the National Parks Service web site. for example, their [page on Cabrillo] lists information on the park, the history, hours, fees, upcoming events, and directions. They have information on all the national parks in the USA.