There are so many wild claims in this post that it's hard to know where to start, but let's start with these.
I'll gladly agree Biden handled the border badly. But the 10-20 million number sounds like something pulled out of thin air. Got a source for that?
And the border bill getting killed wasn't some genius move. Trump wanted the issue alive because it's political gold every election. If the border was actually fixed, Republicans lose one of their biggest talking points.
The entire Illegal population was estimated at 15 million in the US, which was done by some right-leaning institution (forgot which one, will supply that with source if I come across that article again) that estimated the number based on the social support every state gave.
On the other hand, there aren't any border counters you can use to see how many people came across over a specific amount of time (say 4 years); you can calculate or estimate the number that were already in the US when you conducted the study, but there isn't pretty much any way you can estimate correctly how many came through from the Biden administration.
Most CBP figures are about traffic rate; yes, technically they can get that number in 4 years if EVERY TRUCK and CAR that goes over the border carries a full load of illegal migrants, but in reality IIRC, it's like 30-40% of those carry illegal. I know, as I went to FLETC. Which train Federal CBP agents.
That number is a scare tactic; I can tell you this: before I went to FLETC in late 2000, there were already 9 million or so illegal immigrants living in the US. If Biden let in 15 million or 20 million, 1/10 of the US would have been illegal aliens, and 1/3 of the United States would have been Hispanic. Consider that about 25% of the US is Hispanic.
Also, for that figure to work, you need 600 Mexicans to cross the border every hour every day for 4 years (20,000,000/4/365/24). That's pretty much impossible.