There are studies that show that immigration leads to higher job and wage growth: https://www.migrationpolicy.org/explainer/immigrants-us-econ... (look at the bottom for sources). It's not an assumption.

You're focusing on one side of the supply-demand graph. Immigrants increase both the demand & supply side, they require more jobs but also the increase in population increases demand for more products and services. There is no fixed amount of jobs in a country.

I do agree if a country were to let in 100 million immigrants over night or some insane number that it would mess up the economy, there is an upper bound to this.