Quesion of elision

La, what you are referring to is ‘liaison’ - it is optional in many cases, as in the examples you refer to. 

Optional liaisons are less frequently used in everyday French nowadays than previously. There are a limited number of ‘forbidden’ liaisons and obligatory liaisons. 

The series of articles linked are helpful, defining and describing the technical aspects, but you will most likely just develop a feel for usage over (a long) time !

 https://progress.lawlessfrench.com/revision/glossary/pronunciation/liaison

 https://www.lawlessfrench.com/pronunciation/liaisons/

 https://www.lawlessfrench.com/pronunciation/required-liaisons/

 https://www.lawlessfrench.com/pronunciation/optional-liaisons/

 https://www.lawlessfrench.com/pronunciation/forbidden-liaisons/

Elision relates to obligatory contractions of words that are required in formal speech and writing.

‘Informal elisions’, referred to by various names across French teaching sites to try to distinguish from the formal elisions, are also very frequent in everyday speech. 

https://progress.lawlessfrench.com/revision/glossary/pronunciation/liaison

https://www.lawlessfrench.com/pronunciation/elision/

 https://www.lawlessfrench.com/grammar/informal-pronouns/