There are many ways to do it. On my Cayo, I solvent welded the flanges to the tanks. It is rare for a flange to fail, and if it does there are various alternatives involving cutting out some or all of the old flange. On the other hand, that doesn't work with polyethylene tanks, only with ABS tanks.
I don't see any sealant in your photo, except for some accumulated debris on the valve.
To learn to see below the surface, you must adjust your altitude