Copper is quite easy to sweat. The surfaces must be clean, steel wool works great, so does fine emery cloth. A good flux and plenty of heat, preferably MAP gas.
Heat the joint uniformly. When hot touch the solder to it, when it begins to flow it should suck right into the joint, all the way around. Push the solder into the joint as it flows and stop after the first drip on the bottom.
A good plumber does not wait for the first drip to stop, but can tell when the joint is full by the silver ring that develops all the way around.
I’ve actually done quite a bit of copper pipe plumbing, years ago, but always the parts LOOKED like they needed to be sanded or steel wooled. See also my above comments about copper wire (stranded copper wire, actually) vs. this copper tubing.
What threw me a bit is seemingly similar copper surfaces (the wire vs. this tubing, both looking “clean” / fresh) having a much different requirement to solder to it.