Specifically, can an Ultegra r6800 11-speed, 11t sprocket replace the 11t sprocket on a 105 r7000 cassette without issue?
All 11-tooth Shimano and SRAM sprocket combinations are interchangeable for as long as they do not (heavily) exceed the derailleur range and capacities. Shimano and SRAM also share the same freehub body.
This is also true for the 10-speed combinations.
The only differences are the price, the weight and the material, either steel, aluminium, titanium or carbon fibre, but these don't affect the function.