Still I am using Apple Iphone 6 plus mobile it's IOS version is 12.5.5 I used secondhand mobile so i can't download youtube,facebook amd some apps what i need because this apps request higher IOS version so can't you help me to download this apps

You need a newer iPhone.

An iPhone 6 cannot updated past iOS 12.5.6. That is the last version it supports. If apps you want require a newer iOS version, then you'll need a newer iPhone that can run that iOS version or newer iOS version.

If you have downloaded it before with that same Apple ID, you may be able to get a compatible version from your history: click here ➜ Redownload apps and games from Apple - Apple Support

If not, then there's no way to download Youtube to that iPhone model anymore.

You can see if you can still access those sites using a web browser.

It is possible to survive without those apps. ;-) I don't have a Facebook account at all and I only watch YouTube with a browser.

Then you won't be able to download Youtube.