What cellard has said is partially right. Halving just increases eagerness among the panic new crypto investors who feel like the btc which needs to be mined is very less in number and hence they start panic investments which drives the price higher than usual during the halving year.
i wouldn't call it "panic investment" it is a speculation and it exists in every other market too. for example if you speculate that a company is going to expand this year and perform a lot better, you start buying its stock and if enough people have the same speculation the price of that company's shares goes up. same is true about gold, oil, real estate,... and bitcoin.
This increase in interest among the panic guys carry over to the next year and we experience a steep rise in price similar to what happened last year. This rise in price won't sustain for a longer period unless btc acceptance increases in other countries.
the follow up rise that happens after the halving is a slow rise and is because of the adoption is happening while the rate at which the supply was being created reduced. in simple terms there are more buyers than there are sellers after the halving.
If you are an investor and only invest your money in btc, you can never earn a decent amount in crypto.
i don't know what your definition of "decent amount" is but going from $1000 to $20000 aka a 1900% profit in 1 year is more than "decent amount"!
Diversifying your portfolio by investing in alts probably will increase your investment price and later converting them to btc will make you earn quite a worthy bucks.
no it won't unless you are an experienced altcoin trader who knows how to make profit from altcoin pump and dumps. otherwise you are gambling and there is a good chance of losing a lot of money.