what happens if I switch the cloud on for my old photo app

You haven't told us what Mac and iPhone you are using, and you haven't told us what OSs they are running. Also, "trying to download my phone photos" could mean different things, so it's hard to answer your questions.

I understand that you have pictures on your iPhone and pictures in iCloud Photos, and that you want them combined. Do you have enough storage space on your iPhone, on your Mac, and in iCloud Photos to contain them all? Are the Mac, iPhone, and iCloud signed in to the same Apple ID? Do you use an external drive to hold your Mac Photos Library, or do you keep the Mac Photos Library in the Pictures folder of your Mac?

We don't know if you use an external drive, but if you do, how is it formatted? Here is a warning: To avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. Additionally, the drive can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:

Move your Photos library to save space on your Mac - Apple Support

So, if you have enough space, and you're using Mac formatted drives, then here's you could do: Turn on iCloud on all three devices. When you do that, all the pictures on your phone, and all the pictures in your Mac Photos, will be copied to iCloud Photos. Then all the pictures that weren't in the Mac Library will be copied there, and all the pictures that weren't on your phone will be copied to it.

iCloud Photos doesn't remove any pictures, but it makes sure that all the Libraries are identical.

If you don't have enough room to hold all the pictures on your phone, you can go to your iPhone Settings>Photos and check "Optimize Storage" in order to save lots of space. This will keep mostly smaller images on your phone, and your phone will access full sized images in iCloud Photos when needed. You can do this on your Mac as well. Especially on the Mac it's better to "Copy Originals," though that takes up more space, and that's why some people use external drives. But you can choose "Optimize Storage" for now, and deal with that later.

Well, that's what I'd do, anyway. More questions?

If you have enough space for your photos in the cloud, it would be a smart idea to upload them there. Please keep in mind that iCloud is not a true backup/archive service. It is a syncing service, so one you start using iCloud for photos, if you choose to remove a photo on any device, which is signed into the same Apple ID, the photo will delete everywhere. And yes, you can access the photos in iCloud by going to www.icloud and signing in with your Apple ID and Password.

The most important thing for you to understand is that iCloud is a syncing service, not a true Archive. What this means is that changes you make on any device you are syncing photos with iCloud, will be mirrored on ALL devices synced with iCloud. Example: On your iPhone, if you delete a photo, it will delete everywhere, from iCloud and any other device you sync photos from.

G Martin wrote: if i understand it all, those images are still in the cloud. not on my laptop.

this is all so confusing.

You didn't say what macOS your are running.

If your iPhone is connected to iCloud, then whatever pictures are on the phone are also at iCloud Photos. Whatever pictures are at iCloud Photos are also on your phone. If your Mac has iCloud turned on , then whatever pictures are on the Mac are also at iCloud Photos. Whatever pictures are at iCloud Photos are also on your Mac.

iCloud Photos is a synchronization service. It keeps all the pictures on all the devices the same. If you take a picture with your phone, then that picture is added to iCloud and copied to the Mac. If you add a picture on your Mac, then that picture is copied to iCloud and then copied to your phone. If you delete a picture on your phone, then it is deleted everywhere. There is no need for cables to connect things-- it all happens automatically over wifi and the internet.

The pictures are stored in a Library, not in an app. The Photos app has the instructions to organize, edit, and display the pictures and their data that are in the Photos Library. The app is in the Applications folder, and the Library is in the Pictures folder.

An iPhone and an iPad can only have one Library. iCloud has only one Library for your account. But a Mac is allowed to have multiple Libraries, although only one of them, called the System Library, can be connected to iCloud. Most people have only one Photos Library on their Macs, and that Photos Library is stored in the Pictures folder.

I threw a lot of stuff at you, hoping it might make things clearer. (If it makes it muddier, try to forget it!) There's stuff about Photos here:

Set up and use iCloud Photos - Apple Support

This is more about step by step details than offering an overview of the system, I think.

So, your best bet is to stop connecting cables and to turn on iCloud in Photos on your Mac. And then let it all do its thing.

Let us know how it's going...

The original files are stored in the iCloud Photos LIbrary. If you have "Download Originals" checked on a device, then they are also stored in that device's System Photos Library. It's certainly possible (if no device has "Download" checked) that there is no device, other than iCloud, that has the original files.

If a picture originates on your iPhone, but you don't have "Download Originals" checked there, then the originals will NOT be on the originating Library.

If a device has has had "Download Originals" checked for a long time (to be sure,) then that device's Library can be copied to another drive, and then that drive will also have all the originals, even though it's not a System Library. That's how we make backups for our Libraries.

G Martin wrote:

got it, thank you.

so the originals would be stored in the system photo library of the device that originated either the photo or the file?

Yes