What is difference between ubuntu 18 and 20? [closed]

Ubuntu has different products.

Snap only products - year (18)

The Ubuntu products using the year format are snap only, eg Ubuntu Core 18 is the 2018 release of Ubuntu Core for devices, appliances, but can also be used in cloud environments. Ubuntu Core 20 is the 2020 release of the Ubuntu Core product. All these products come with 10 years of supported life, so LTS is not used.

Main deb products - year.month (18.04)

The Ubuntu products using the year.month format are the main deb based releases, showing the year & month of release, eg. Ubuntu 20.04 LTS is the 2020-April release of Ubuntu; the 20.04 format tells you it's deb package based and the LTS shows you it's a long-term-support release with 5 years of support (3 for flavors like Lubuntu). Non-LTS releases have 9 months of supported life, and LTS products can have the life extended via the use of ESM. These products can also use snap packages.

Lubuntu is a desktop, and does not have year or snap only releases, so Lubuntu 20.04 LTS has the LXQt desktop, is built on the Ubuntu 20.04 platform. It's apps & desktop differs to the Ubuntu 20.04 Desktop system (LXQt & Qt5 apps replaced the GNOME & GTK3 apps found on Ubuntu Desktop).

Lubuntu 18.04 LTS (or the 2018-April release) has completed it's 3 years of supported life, but it used the older & deprecated LXDE environment, built on the (deprecated) GTK2 toolkit. It was the last Lubuntu release that used the LXDE desktop. Ubuntu 18.04 LTS Desktop using the GNOME desktop however has five years of supported life, so it still receives upgrades & security fixes.