In a sideways market...

Buyers and sellers are evenly matched

One huge green candle appears in a downtrend. What does it prove?

Not much: one candle isn't a trend

Prices never move in straight lines. They zigzag. A trend is the general direction of those zigzags.
Uptrend: each peak is higher than the last, and each dip stops higher than the last. Higher highs, higher lows.
Downtrend: lower highs and lower lows. Every bounce fails below the last one. Sellers are in charge.
Sideways: price bounces between a floor and a ceiling. Neither side is winning, and markets spend a lot of time like this.
Real example: from March 2009 to early 2020, the S&P 500 kept making higher highs and higher lows, climbing from under 800 to over 3,000.
Common mistake
Mistake: calling a new trend from one big candle. One candle is a moment; a trend needs a series of higher (or lower) peaks and dips.
Key takeaway
Up: higher highs and higher lows. Down: lower highs and lower lows. Neither: sideways. Read the trend before anything else.

Higher highs and higher lows is called... An uptrendPeaks and dips that each stop higher than the last: that's an uptrend by definition.
In a sideways market... Buyers and sellers are evenly matchedPrice bounces between a floor and a ceiling because neither side can win.
Why zoom out on a chart? To see the bigger trendA dip on a short chart can sit inside a big uptrend you only see when you zoom out.
One huge green candle appears in a downtrend. What does it prove? Not much: one candle isn't a trendTrends are a series of peaks and dips. One candle can't make that series alone.