After I bought an Android phone from a second-hand store, as i know it was 64 GB, but I checked the actual memory for 20 GB. and there are not many photos,messages and apps on this Android phone.Maybe they were saved in a device file on the phone, and I couldn't find them.How do I deal with them?

the android operating system should show you waht is whaere and how much of it is used, iirc under settings => storage.
if it doesn't show 64GB, i find it likely that you were cheated.

I wish I'd been cheated, if so that i can return it , but it shows 64 GB ,the available memory is only 20 GB,That means that 44 GB has been taken up.

The OS is stored on that 64GB, if it's like a tablet, therefore it is 64 minus the size of OS, for your actual storage space.

Somewhere in the Android settings (it tends to vary a bit from version to version) there is a place to check your storage usage. On my phone (T-Mobile HTC U11 with Android v. 8.0.0), it's Settings-->Phone-->Storage.

Sure ,maybe i need to clean up my phone space ,I suspect there's still some data or apps on the phone that hasn't been cleaned up.
That's the only way to do that

It is ALWAYS a good idea to factory reset a second hand phone. I always boot into recovery first thing and totally wipe it.

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Totally agree, that's the first thing I'd do. In many countries it's illegal for a store to sell a phone that hasn't been factory reset.

Lacemrt: If you didn't factory reset the phone yourself and have used it with any of your own accounts I would strongly suggest resetting the password on all those accounts and NOT from the phone. You have no idea whether or not any spyware was running.

Can you make sure that Factory reset can completely delete them from my phone , But it seems unlikely.

a factory reset will delete EVERYTHING from the phone that wasn't on it when it came from the factory. hence the name.

I don't know if you have tried ,but i have tried once , the data can be deleted form my phone after the factory reset , but they don't seen to have been completely delete since i can recover them back ,

totally agreed

Just personal story.

I bought a SAMSUNG SM-T580 tablet.

SAMSUNG refuses to help me in any way in regards of support.

the device claims to have 16gb of Flash.
IT had around 9gb of free.

after trying everything i voided my warranty in replacing the operating system. i have now ~11GB free

The issue with SAMSUNG ANDROID is a stupid warranty policy, fake claims of octa core prozessors and specs, bloatware and stupid customisation. fake octacore is because it is a dual quadcore with different max speed ratings, different arichitecture, different power consumption. I suspect the device has around 14gb of available rom only. typical samsung fraud, as publically known.

It is always advised to get a phone with google supported update, afaik they named it android one?? or with lineage os supported device

samsung support told me in german for my country, thad i could have informed me before purchase at the homepage and now i have bad luck. I have those e-mails in germans for reference!

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quote: a factory reset will delete EVERYTHING from the phone that wasn't on it when it came from the factory. hence the name.

Nope
You bought a phone with undefined state and undefined operating system
I would only trust a phone where I could for example flash myself the full operating system. E.g. nexus 4 + downloaded OS from google itself

Anyone can build these days android operating system and than you just need some startup script which just is enabled by default. even a factory reset will not prevent this. as you are not able to look into all the file system on default on android, you can not check anything. and even that does not prove that some things can be hidden. xdadevelopers shows regularly ways on how to fool games, or other apps to think everything is in a sane state.

An android device is not secure by design. some manufacture just replaced the security patchset date without doing any patching at all. other devices just phone home sensible data to china or elsewhere.

+ risks of hidden things in firmware which is not covered by flashing the device. cpu bugs e.g. nvidia switch and such ..

Just be careful on what you do with your android based device. I suggest no electronic banking at all.

so what is your question? Free up more space on the phone? If yes, then you should check this guide, which tells you how to get more free space on Android based devices.

Thanks for you again