Actual good answers here so i'll do a shit tl;dr to even it out

tl;dr of why - to make Line Go Up. They run out of space to further expand "market saturation" and workers/resources to exploit in their home country and in order to make line keep going up instead of just staying steady, they are forced to expand and continue to look for cheap resources and labor to keep cutting out the bottom line.

How they actually do is variable. It can be through political influence like influencing the home country to do a war or open up trade or just straight up funding illegal expansions in other countries. More recently, these big corporations just essentially bribe the target country with "it'll create jobs" and legal political lobbying. You can look up the history of virtually any multi-national corporation and they'll follow the same pattern. get big in home country --> use circumstances/political influence to expand in other territory while continuously ghoulish shit because capitalism necessitates doing that to cut out the costs of the bottom line. And since these companies aren't coming over for benign reasons like actually genuinely fair trade - they are coming to expand and exploit, well that's imperialism bby.