Dear Mover123

I dont see how you will stretch and not stretch either your tendons or ligaments. They are all part of the musculoskeletal complex... the only way you can stretch the ligaments to a point of laxity is if you take it out of its anatomical limit. This is variable for everyone...some people are hypermobile, others are hypomobile, some are in between...
taken a joint out of its anatomical limit will put a dangerous stretch on the ligaments...usually a brute force is needed...i doubt if you can easy do this yourself without meaning to cause harm...

I dont see how you can stretch the muscle belly and not stretch the tendon without having some manual help from somebody else to stabilize where the tendon joins with the muscle....

It is good to have strong and flexible tendons as well as ligaments...too much tightness in them can lead to problems in other places so stretching can be generally beneficial...we all do this naturally everyday in our daily activities, whether you are reaching up a high surface, waking up in the morning...

imagine walking around with extremely tight ligaments o tendons everywhere...you would hate yourself....

Usually the advice is to stretch gradually not forcefully, to allow things elongate over a period of time if that is your aim...trying to stretch vigorously can lead to a tendon/muscle tear/avulson fracture especially if that tendon is not as flexible...

whoever gave you the advice that you should stretch without stretching your ligamnets or tendons doesnt know what they are talking about...