Looking for ways to control a website and it's elements with java when website is in html (obviously)?

I am unfortunately a total noob in programming and only know very basic stuff like loops, classes, etc.

No real life programming yet.

so the thing is:

There is a certain website I'm interested too.

let's say for example I want my program to open the browser with that page, search for and click a specific button, do some other stuff with site elements there, etc.

you know, the kind of html elements used in the website, button, areas, etc.

I wanna manipulate them as in clicking a certain button (which can be anywhere on the site, just finding it with some button id or stuff) write stuff in a specific field, etc.

How do i do that?

I have seen or rather used many greesemonkey scripts for autoclickers on website that obviously dont really just click on specified spots but actually search page for specific elements and use them by somehow calling some appropriate functions.

like "setFieldValue" and stuff like that.

But how on earth does one do that? How do I access website elements with java and call them, use them, use their given functions, etc.?

like going to a website, typing something in the text field and lick the sent button. just not with autoclicking, but by calling the text fields change value function and calling the buttons press-button function.

but how can this work, given that my program would be in java but the website is in html or even php?

where can i learn about this stuff?

obviously heard the word API soemtimes which is used when some no-programmers create the greatest games by linking all tons of different sites, apps, etc. with each other despite them all being in different programming languages.

but I doubt that is what I would use in my case.