How do these scientists explain it when the ‘sea level’ goes UP in one area, and DOWN in another ?
That article goes a long way towards answering your question. But the best article I know of that explains how sea level is accurately measured, which precisely answers your question, is this one...
Claim That Sea Level Is Rising Is a Total Fraud
Oh, FWIW, the tides are not the same every day at the same times of the day. In fact they vary quite a bit. Check the Farmer's Almanac for tide charts.
Heavily glaciated lands rise after the glaciers melt and remove the weight of the ice. This might be interpreted as a drop in sea level.