Gear shifting : new to cycling maintenance
There is a number of things that can result in bicycle gears 'auto shifting'.

The gear indexing could be out of adjustment, so that the chain is either getting caught by the next biggest sprocket causing a down-shift, or falling off onto the next smallest sprocket causing an up-shift. If the indexing is off enough to cause auto-shifting the chain would be making a lot of noise though.

Cables and housings may be dirty, worn or loose which causes the input at the shifter to not appear accurately at the derailleur.

There are many articles and YouTube videos available that will teach you bike mechanics (my personal favorites are from Park Tool). However, if you are not experienced with bike mechanics you will probably be best taking your bike to a good local repair shop. Ask the tech to explain what their diagnosis is and their recommended fix.

Update based on extra info:

New bike with ~200m very likely needs a post break-in tune up including adjustment of the gear indexing to account for cable stretch. Bike shops in the US at least (the extent of my personal experience) commonly do this for free,