Hi

On 31 May 2003 the Landsat 7 Enhanced Thematic Mapper (ETM+) sensor had a failure of the Scan Line Corrector (SLC). Since that time all Landsat ETM+ images have had wedge-shaped gaps on both sides of each scene, resulting in approximately 22% data loss. These images are available for free download from the USGS GloVis website and are found in the L7 SLC-off collection.

source: Yale University

Hello Salauat Kalabaev

To fill in the gaps and blur in Landsat 7 images, you can use several techniques:

Read here:

https://gis.stackexchange.com/questions/30250/how-to-fill-gaps-in-slc-off-landsat-7-etm-images-with-qgis

https://yceo.yale.edu/how-fill-gaps-landsat-etm-images

As for your second question, it depends on your specific needs and the quality of data you require.

• Level-1 Landsat 7 images are delivered as quantized and calibrated scaled Digital Numbers (DN). The Level-1 DN data can be rescaled to Top-of-Atmosphere (TOA) reflectance by applying radiometric rescaling coefficients. If you require the most recent satellite data, you will be limited to Level 1 data.
• Level-2 Landsat 7 images contain surface reflectance values, i.e., reflectance as it would be measured at ground level in the absence of atmospheric effects. If you plan to use the best pre-processed (Surface Reflectance) Landsat data, it is recommended to use Collection 2 Level 2 Tier 1.

Read here:

https://www.usgs.gov/landsat-missions/using-usgs-landsat-level-1-data-product

https://www.usgs.gov/landsat-missions/landsat-collection-2-level-2-science-products

Best Regards,

Ali YOUNES