To achieve that, you need to use key_down and key_up methods to press and release the keys in the correct chain order:
from selenium.webdriver.common.action_chains import ActionChains
from selenium.webdriver.common.keys import Keys
...
actions = ActionChains(driver)
actions.click(elementimclickingon).key_down(Keys.CONTROL).send_keys("a").key_up(Keys.CONTROL).perform()
Here is a full minimal example to select all text with Ctrl + A and copy it with Ctrl + C:
from selenium import webdriver
from selenium.webdriver.common.action_chains import ActionChains
from selenium.webdriver.common.keys import Keys
from selenium.webdriver.common.by import By
driver = webdriver.Edge()
driver.get("http://www.example.com")
element = driver.find_element(By.TAG_NAME, "body")
actions = ActionChains(driver)
# Ctrl + A
actions.click(elementimclickingon).key_down(Keys.CONTROL).send_keys("a").key_up(Keys.CONTROL).perform()
# Ctrl + C
actions.key_down(Keys.CONTROL).send_keys("c").key_up(Keys.CONTROL).perform()
driver.quit()
If you only want to send a "select all" command, you can use JavaScript for that with execute_script like this:
driver.execute_script("document.execCommand('selectAll')")